Based on the man page, calendar(3) would not suit my use well. It's quite different from calendar.txt. But I'm happy to hear that you use it.

I installed it just to read the man page: """

           LANG=C
           Easter=Ostern

           #include <calendar.usholiday>
           #include <calendar.birthday>

           6/15\tJune 15 (if ambiguous, will default to month/day).
           Jun. 15\tJune 15.
           15 June\tJune 15.
           Thursday\tEvery Thursday.
           June\tEvery June 1st.
           15 *\t15th of every month.

           May Sun+2\tsecond Sunday in May (Muttertag)
           04/SunLast\tlast Sunday in April,
           \tsummer time in Europe
           Easter\tEaster
           Ostern-2\tGood Friday (2 days before Easter)
           Paskha\tOrthodox Easter

"""