I wrote some code for finding the date of Easter. Now I want to use it with the TiKZ calendar to mark the Easter related holidays.
Here's what I have so far:
\documentclass{scrartcl}
\usepackage[T1]{fontenc}
\usepackage{libertine}
\usepackage[margin=5pt,a5paper,landscape]{geometry}
\setlength{\parindent}{0pt}
\usepackage{tikz}
\usetikzlibrary{calendar}
\usepackage{expl3}
\ExplSyntaxOn
%% 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
\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
}
}
\ExplSyntaxOff
\begin{document}
\begin{tikzpicture}
\calendar[dates=2012-04-01 to 2012-05-last, week list];
\end{tikzpicture}
\end{document}