\usepackage{graphicx}
\usepackage{tikz}
%%%<
\usepackage{verbatim}
%%%>

\usetikzlibrary{calendar,fit}
\usepackage{expl3,xparse}

%% Adapted from http://tex.stackexchange.com/a/10199/4771
\makeatletter%
\tikzoption{day headings}{\tikzstyle{day heading}=[#1]}
\tikzstyle{day heading}=[]
\tikzstyle{day letter headings}=[
    execute before day scope={ \ifdate{day of month=1}{%
      \pgfmathsetlength{\pgf@ya}{\tikz@lib@cal@yshift}%
      \pgfmathsetlength\pgf@xa{\tikz@lib@cal@xshift}%
      \pgftransformyshift{-\pgf@ya}
      \foreach \d/\l in {0/Montag,1/Dienstag,2/Mittwoch,3/Donnerstag,%
                         4/Freitag,5/Samstag,6/Sonntag} {
        \pgf@xa=\d\pgf@xa%
        \pgftransformxshift{\pgf@xa-\cellwidth/2}%
        \pgftransformyshift{\pgf@ya}%
        \node[above=-0.5ex,day heading]{\l};%
      }
    }{}%
  }%
]
\makeatother%
%% End

%% Adapted from pgf source
\def\pgfcalendarmonthname#1{%
  \translate{\ifcase#1\or Januar\or Februar\or März\or April\or
    Mai\or Juni\or Juli\or August\or September\or Oktober\or
    November\or Dezember\fi}%
}

\makeatletter
\tikzstyle{month label above centered}=[%
  execute before day scope={%
    \ifdate{day of month=1}{%
      {
        \pgfmathsetlength{\pgf@xa}{\tikz@lib@cal@xshift}%
        \pgf@xb=\tikz@lib@cal@width\pgf@xa%
        \advance\pgf@xb by-\pgf@xa%
        \pgf@xb=.5\pgf@xb%
        \pgftransformxshift{\pgf@xb}%
        \pgftransformxshift{-\cellwidth/2}%
        \pgfmathsetlength{\pgf@y}{\tikz@lib@cal@yshift}%
        \pgftransformyshift{0.333\pgf@y}
        \tikzmonthcode%
      }
    }{}},
  every month/.append style={anchor=base}
]
\makeatother
%% End

\ExplSyntaxOn

%% Adapted from http://tex.stackexchange.com/a/56214/4771
%%
% first of all we define the user level commands
\NewDocumentCommand{\addtext}{ m }{ \bdaycal_input_add:n { #1 } }
\NewDocumentCommand{\addtextyear}{ mm }{ \bdaycal_input_add_y:nn { #1 } { #2 } }
\NewDocumentCommand{\showtext}{ }{ \bdaycal_output_direct: }

% allocate variable:
% a sequence for global storage of the inputs;
\seq_new:N \g_bdaycal_input_seq

% store globally an input in the sequence
\cs_new:Npn \bdaycal_input_add:n #1
 {
  \seq_gput_right:Nn \g_bdaycal_input_seq { #1 }
 }

\cs_new:Npn \bdaycal_input_add_y:nn #1 #2
 {
  \seq_gput_right:Nn \g_bdaycal_input_seq { #1 ~ ( \int_to_arabic:n
    { \pgfcalendarifdateyear - #2 } ) }
 }

% how to output in direct order; we simply do a mapping function calling
% \bdaycal_print:n after incrementing the counter
\cs_new_protected:Npn \bdaycal_output_direct:
 {
  \seq_map_inline:Nn \g_bdaycal_input_seq
   {
    \bdaycal_print:n { ##1 }
   }
  \seq_gclear:N \g_bdaycal_input_seq
 }

% the printing macro; change here for adapting to your wishes
\cs_new:Npn \bdaycal_print:n #1
 {
  #1 \par
 }
%% End

%% Knuth's AoCP, vol 1, 2nd ed, pp 155--156
\int_new:N \l_easter_Y_int
\int_new:N \l_easter_G_int
\int_new:N \l_easter_C_int
\int_new:N \l_easter_X_int
\int_new:N \l_easter_Z_int
\int_new:N \l_easter_D_int
\int_new:N \l_easter_E_int
\int_new:N \l_easter_N_int
\int_new:N \l_easter_M_int
\int_new:N \l_easter_julian_day_int

\cs_new:Nn \easter_sunday:n {

  \int_set:Nn \l_easter_Y_int { #1 }

  \int_set:Nn \l_easter_G_int {
    \int_mod:nn { \l_easter_Y_int } { 19 } + 1
  }

  \int_set:Nn \l_easter_C_int {
    \int_div_truncate:nn { \l_easter_Y_int } { 100 } + 1
  }

  \int_set:Nn \l_easter_X_int {
    \int_div_truncate:nn { 3 * \l_easter_C_int } { 4 } - 12
  }

  \int_set:Nn \l_easter_Z_int {
    \int_div_truncate:nn { 8 * \l_easter_C_int + 5 } { 25 } - 5
  }

  \int_set:Nn \l_easter_D_int {
    \int_div_truncate:nn { 5 * \l_easter_Y_int } { 4 } - \l_easter_X_int - 10
  }

  \int_set:Nn \l_easter_E_int {
    \int_mod:nn { 11 * \l_easter_G_int + 20 + \l_easter_Z_int
      - \l_easter_X_int } { 30 }
  }

  % \int_mod:nn behaves strangely.
  \int_compare:nNnT { \l_easter_E_int } < { 0 }
  {
    \int_add:Nn \l_easter_E_int { 30 }
  }

  \int_compare:nNnTF { \l_easter_E_int } = { 25 }
  { % true
    \int_compare:nNnT { \l_easter_G_int } > { 11 }
    { % true
      \int_incr:N \l_easter_E_int
    }
  }
  { % false
    \int_compare:nNnT { \l_easter_E_int } = { 24 }
    { % true
      \int_incr:N \l_easter_E_int
    }
  }

  \int_set:Nn \l_easter_N_int { 44 - \l_easter_E_int }

  \int_compare:nNnT { \l_easter_N_int } < { 21 }
  { % true
    \int_add:Nn \l_easter_N_int { 30 }
  }

  \int_add:Nn \l_easter_N_int {
    7 - \int_mod:nn { \l_easter_D_int + \l_easter_N_int } { 7 }
  }

  \int_compare:nNnTF { \l_easter_N_int } > { 31 }
  { % true
    \int_sub:Nn \l_easter_N_int { 31 }
    \int_set:Nn \l_easter_M_int { 4 } % April
  }
  { % false
    \int_set:Nn \l_easter_M_int { 3 } % March
  }

  \pgfcalendardatetojulian { \l_easter_Y_int -
     \l_easter_M_int - \l_easter_N_int
  } { \l_easter_julian_day_int }

}

\pgfkeys{/pgf/calendar/Easter/.default = 0}
\pgfkeys{/pgf/calendar/Easter/.code =
  {
    \easter_sunday:n { \pgfcalendarifdateyear }
    \int_compare:nNnT { \pgfcalendarifdatejulian }
    =  {\l_easter_julian_day_int + #1}
    { \pgfcalendarmatchestrue }
  }
}

\int_new:N \l_advent_Y_int
\int_new:N \l_advent_xmas_julian_day_int
\int_new:N \l_advent_xmas_week_day_int
\int_new:N \l_advent_julian_day_int

\cs_new:Nn \advent_sunday:n {

  \int_set:Nn \l_advent_Y_int { #1 }

  \pgfcalendardatetojulian { \l_advent_Y_int - 12 - 25 } {
    \l_advent_xmas_julian_day_int }

  \pgfcalendarjuliantoweekday { \l_advent_xmas_julian_day_int } {
    \l_advent_xmas_week_day_int }

  \int_set:Nn \l_advent_julian_day_int {
    \l_advent_xmas_julian_day_int - \l_advent_xmas_week_day_int - 22 }

}

\pgfkeys{/pgf/calendar/Advent/.default = 0}
\pgfkeys{/pgf/calendar/Advent/.code =
  {
    \advent_sunday:n { \pgfcalendarifdateyear }
    \int_compare:nNnT { \pgfcalendarifdatejulian }
    =  {\l_advent_julian_day_int + #1}
    { \pgfcalendarmatchestrue }
  }
}

%% http://www.tondering.dk/claus/cal/week.php#calcweekno
\int_new:N \l_week_number_year_int
\int_new:N \l_week_number_month_int
\int_new:N \l_week_number_day_int
\int_new:N \l_week_number_a_int
\int_new:N \l_week_number_b_int
\int_new:N \l_week_number_c_int
\int_new:N \l_week_number_s_int
\int_new:N \l_week_number_e_int
\int_new:N \l_week_number_f_int
\int_new:N \l_week_number_g_int
\int_new:N \l_week_number_d_int
\int_new:N \l_week_number_n_int
\int_new:N \l_week_number_W_int

\cs_new:Nn \week_number:nnn {

  \int_set:Nn \l_week_number_year_int { #1 }
  \int_set:Nn \l_week_number_month_int { #2 }
  \int_set:Nn \l_week_number_day_int { #3 }

  \int_compare:nNnTF { \l_week_number_month_int } < { 3 } % jan or feb
  { % true

    \int_set:Nn \l_week_number_a_int { \l_week_number_year_int - 1 }

    \int_set:Nn \l_week_number_b_int {
      \int_div_truncate:nn { \l_week_number_a_int } { 4 }
      - \int_div_truncate:nn { \l_week_number_a_int } { 100 }
      + \int_div_truncate:nn { \l_week_number_a_int } { 400 }
    }

    \int_set:Nn \l_week_number_c_int {
      \int_div_truncate:nn { \l_week_number_a_int - 1 } { 4 }
      - \int_div_truncate:nn { \l_week_number_a_int - 1 } { 100 }
      + \int_div_truncate:nn { \l_week_number_a_int - 1 } { 400 }
    }

    \int_set:Nn \l_week_number_s_int {
      \l_week_number_b_int - \l_week_number_c_int }

    \int_zero:N \l_week_number_e_int

    \int_set:Nn \l_week_number_f_int { \l_week_number_day_int - 1
      + 31 * ( \l_week_number_month_int - 1 ) }

  } % end true
  { % false

    \int_set_eq:NN \l_week_number_a_int \l_week_number_year_int

    \int_set:Nn \l_week_number_b_int {
      \int_div_truncate:nn { \l_week_number_a_int } { 4 }
      - \int_div_truncate:nn { \l_week_number_a_int } { 100 }
      + \int_div_truncate:nn { \l_week_number_a_int } { 400 }
    }

    \int_set:Nn \l_week_number_c_int {
      \int_div_truncate:nn { \l_week_number_a_int - 1 } { 4 }
      - \int_div_truncate:nn { \l_week_number_a_int - 1 } { 100 }
      + \int_div_truncate:nn { \l_week_number_a_int - 1 } { 400 }
    }

    \int_set:Nn \l_week_number_s_int {
      \l_week_number_b_int - \l_week_number_c_int }

    \int_set:Nn \l_week_number_e_int { \l_week_number_s_int + 1 }

    \int_set:Nn \l_week_number_f_int { \l_week_number_day_int
      + \int_div_truncate:nn {
        153 * ( \l_week_number_month_int - 3 ) + 2 } { 5 }
      + 58 + \l_week_number_s_int }

  } % end false

  \int_set:Nn \l_week_number_g_int {
    \int_mod:nn { \l_week_number_a_int + \l_week_number_b_int } { 7 }  }

  \int_set:Nn \l_week_number_d_int {
    \int_mod:nn { \l_week_number_f_int + \l_week_number_g_int
      - \l_week_number_e_int } { 7 }  }

  \int_set:Nn \l_week_number_n_int {
    \l_week_number_f_int + 3 - \l_week_number_d_int }

  \int_compare:nNnTF { \l_week_number_n_int } < { 0 }
  { %true

    \int_set:Nn \l_week_number_W_int { 53
      - \int_div_truncate:nn { \l_week_number_g_int
        - \l_week_number_s_int } { 5 } }

  } % end true
  { % false

    \int_compare:nNnTF { \l_week_number_n_int } > { 364
      + \l_week_number_s_int }
    { % true

      \int_set:Nn \l_week_number_W_int { 1 }

    } % end true
    { % false

      \int_set:Nn \l_week_number_W_int { \int_div_truncate:nn {
          \l_week_number_n_int } { 7 } + 1 }

    } % end false

  } % end false

}

\definecolor{red}{rgb}{0.937254901961,0.16862745098,0.176470588235}

\newsavebox{\flagCH}
\savebox{\flagCH}{
\begin{tikzpicture}
  \fill[red] rectangle (20pt,20pt);
  \fill[white,xshift=7.5pt, yshift=3pt] rectangle (5pt,14pt);
  \fill[white,xshift=3pt, yshift=7.5pt] rectangle (14pt,5pt);
\end{tikzpicture}
}

\pgfkeys{/tikz/week~number/.code =
  {
    \week_number:nnn {
      \pgfcalendarifdateyear } {
      \pgfcalendarifdatemonth } {
      \pgfcalendarifdateday }
    \addtext{ Woche ~ \int_to_arabic:n { \l_week_number_W_int } }
  }
}

\ExplSyntaxOff

\pgfkeys{/tikz/flag-flying day/.code =
  {
    \draw (-\cellwidth,0) node [above right,font=\Huge]
    {\resizebox{!}{0.8ex}{\usebox{\flagCH}}};
  }
}

\pgfkeys{/tikz/observance/.code =
  {
    \addtext{#1}
  }
}

\pgfkeys{/tikz/anniversary/.code 2 args=\addtextyear{#1}{#2}}

\pgfkeys{/tikz/day code =
  {
    \node (lower right) at (0,0) [above left,font=\Large] {\tikzdaytext};
    \node (upper left) at (-\cellwidth,\cellheight)
    [below right,align=left,text width=\cellwidth-\pgflinewidth,
    font=\tiny,black] {\showtext};
    \node (lower left) at (-\cellwidth,0) {};
    \node[draw,
          fit=(lower right) (upper left) (lower left),
          inner sep=1mm] {};
  }
}

\pgfkeys{/tikz/inner sep = 0pt}

\pgfkeys{/tikz/day xshift=\cellwidth+2mm+2mm}

\pgfkeys{/tikz/day yshift=\cellheight+2mm+2mm}

\newlength{\cellheight}
\setlength{\cellheight}{14mm}
\newlength{\cellwidth}
\setlength{\cellwidth}{24.75mm}
