SlackBuilds.org/office/gcal

Gcal is a program for calculating and printing calendars. Gcal displays
hybrid and proleptic Julian and Gregorian calendar sheets, respectively
for one month, three months, or a whole year. It also displays eternal
holiday lists for many countries around the globe, and features a very
powerful creation of fixed date liststhat can be used for reminding
purposes. Gcal can calculate various astronomical data and times of the
Sun and the Moon for pleasure at any location, precisely enough for most
civil purposes. Gcal supports some other calendar systems, for example,
the Chinese and Japanese calendars, the Hebrew calendar, and the civil
Islamic calendar, too.