I built one of these for myself in 1995 and have been happily using it ever since. Here's the source code: 0 0 19 7 * /usr/bin/mail -s "REMINDER: john T. birthday" me@mydomain.com 0 0 1 8 * /usr/bin/mail -s "REMINDER: MAKE xmas hotel reservations NOW for good pricing..." me@mydomain.com
Date calculations are much more flexible and the files can be modular.. and there's printed calendar generation options if you so choose.
https://www.roaringpenguin.com/products/remind
There is also the traditional Unix 'calendar' program as well..