TikZ calendar, multi-day solid lines like Google Calendar

Question

I'm starting with these examples:

• https://stackoverflow.com/questions/2137597/how-to-create-a-highlighted-calendar-using-latex
• http://www.texample.net/tikz/examples/changing-the-default-calendar-layout/

I'm looking to add a red bar under and possibly through the numbers indicating a multiday event. I'm interested in how to do this both within a week, and also over weekends (where wraparound would be an issue).

Answer 1

This is not an answer but an attempt to clarify your question. All I did was to copy the code from your second link and to add some basic thingy that I adapted drom p. 526 of the pgfmanual. This allows me to draw a red bar.

\documentclass{article}
% An example how to use the calendar library and modify the layout, i.e. put
% Sunday as the first week day.
%
% Author: Berteun Damman
\usepackage{tikz}
\usetikzlibrary{calendar}
\begin{document}
    \makeatletter

    % This way you can define your own conditions, for example, you
    % could make something as `full moon', `even week', `odd week',
    % et cetera. In principle. The math in TeX could be hard.
    \pgfkeys{/pgf/calendar/start of year/.code={%
        \ifnum\pgfcalendarifdateday=1\relax%
            \ifnum\pgfcalendarifdatemonth=1\relax\pgfcalendarmatchestrue\fi%
        \fi%
    }}%

    % Define our own style
    \tikzstyle{week list sunday}=[
        % Note that we cannot extend from week list,
        % the execute before day scope is cumulative
        execute before day scope={%
               \ifdate{day of month=1}{\ifdate{equals=\pgfcalendarbeginiso}{}{
               % On first of month, except when first date in calendar.
                   \pgfmathsetlength{\pgf@y}{\tikz@lib@cal@month@yshift}%
                   \pgftransformyshift{-\pgf@y}
               }}{}%
        },
        execute at begin day scope={%
            % Because for TikZ Monday is 0 and Sunday is 6,
            % we can't directly use \pgfcalendercurrentweekday,
            % but instead we define \c@pgf@counta (basically) as:
            % (\pgfcalendercurrentweekday + 1) % 7
            \pgfmathsetlength\pgf@x{\tikz@lib@cal@xshift}%
            \ifnum\pgfcalendarcurrentweekday=6
                \c@pgf@counta=0
            \else
                \c@pgf@counta=\pgfcalendarcurrentweekday
                \advance\c@pgf@counta by 1
            \fi
            \pgf@x=\c@pgf@counta\pgf@x
            % Shift to the right position for the day.
            \pgftransformxshift{\pgf@x}
        },
        execute after day scope={
            % Week is done, shift to the next line.
            \ifdate{Saturday}{
                \pgfmathsetlength{\pgf@y}{\tikz@lib@cal@yshift}%
                \pgftransformyshift{-\pgf@y}
            }{}%
        },
        % This should be defined, glancing from the source code.
        tikz@lib@cal@width=7
    ]

    % New style for drawing the year, it is always drawn
    % for January
    \tikzstyle{year label left}=[
        execute before day scope={
            \ifdate{start of year}{
                \drawyear
            }{}
        },
        % Right align
        every year/.append style={
            anchor=east,
        }
    ]

    % Style to force giving a month a year label.
    \tikzset{draw year/.style={
        execute before day scope={
            \ifdate{day of month=1}{\drawyear}{}
        }
    }}

    % This actually draws the year.
    \newcommand{\drawyear}{
        \pgfmathsetlength{\pgf@x}{\tikz@lib@cal@xshift}%
        \pgftransformxshift{-\pgf@x}
        % \tikzyearcode is defined by default
        \tikzyearcode
        \pgfmathsetlength{\pgf@x}{\tikz@lib@cal@xshift}%
        \pgftransformxshift{\pgf@x}
    }

    \makeatother

    % The actual calendar is now rather easy:
    \begin{tikzpicture}[every calendar/.style={
            month label above centered,
            month text={\textit{\%mt}},
            year label left,
            every year/.append style={font=\Large\sffamily\bfseries,
                green!50!black},
            if={(Sunday) [blue!70]},
            week list sunday,
        }]
        \matrix[column sep=1em, row sep=1em] {
            \calendar[dates=2018-04-01 to 2018-04-last,draw year]; &
            \calendar[dates=2018-05-01 to 2018-05-last]; &
            \calendar[dates=2018-06-01 to 2018-06-last]; \\
            \calendar[dates=2018-07-01 to 2018-07-last]; &
            \calendar[dates=2018-08-01 to 2018-08-last]; &
            \calendar (mycal) [dates=2018-09-01 to 2018-09-last]; \\
            \calendar[dates=2018-10-01 to 2018-10-last]; &
            \calendar[dates=2018-11-01 to 2018-11-last]; &
            \calendar[dates=2018-12-01 to 2018-12-last]; \\
            \calendar[dates=2019-01-01 to 2019-01-last]; &
            \calendar[dates=2019-02-01 to 2019-02-last]; &
            \calendar[dates=2019-03-01 to 2019-03-last]; \\
        };
    \draw[red] (mycal-2018-09-20.south west) -- (mycal-2018-09-22.south east);  
    \draw[red] (mycal-2018-09-23.south west) -- (mycal-2018-09-24.south east);  
    \end{tikzpicture}
\end{document}

Is your question how to replace the two draw commands by a single macro?