Easter

Easter is a holiday with no fixed date, and therefore it used to be a source of constant confusion to calendar-makers.

Easter comes from Eostre, the name of the Teutonic goddess whose festival was celebrated in the Spring.

Rules for fixing the date of Easter for the Church go back to the fourth century AD, at the Council of Nicaea, where the church met to solve once and for all, some of its problems. Later on, in the 16th century, Pope Gregory XIII proclaimed that Easter would be... "the first Sunday" following the "Full Moon" following the "Spring Equinox".  Say that three times real fast!

And just so there'd be no confusion, the date of the Full Moon, he said, would be derived from a set of ancient lunar tables, and the Spring Equinox he defined with a Papal Bull as being March 21st.

The dates he ascribed for these events, however, do not always match the ones that are calculated astronomically. But no matter, thanks to Pope Gregory XIII, the date of Easter is now easy to figure.