One of the most useful programs that I have used on Palm OS is Holidays & Holidates for Palm.
by Soft-4-Biz
Holidays and Holidates from 2006 to 2015.
Christian, Islamic, Jewish.
72 countries.
Install/Uninstall events to Memos or Calendar.
Runs on all Palm handhelds with OS 3.5 and higher.
Customize date formats, coutries list etc.
Logical and intuitive interface.
Free upgrades.
More deets and download.