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
Very recently, I tried using a combination of nail(1), and calendar(1) on OpenBSD as an email and calendaring combination for home use. "nail" is provided by the OpenBSD s-nail package and provides an extended POSIX mailx style environment. calendar(1) is part of the base OpenBSD distribution.
It worked far better than expected. The only thing missed was age calculations for birthdays. I never figured that out. Using the Emacs Diary provided that sort of thing though.
I've since moved to KDE Kontact for mail/calendaring. It works more smoothly if you do a lot of calendar sharing with others, but I could see going back to nail(1)/calendar(1) if Kontact disappoints.
I installed it just to read the man page: """
"""