### Clue: Eve of historic event in 1916, for which date's varying with year (6,3)
### Answer: EASTER DAY 
 ### Explanation: Definition: Eve of historic event in 1916
The Irish Easter Rising began on Easter Monday, 1916; and the date of Easter varies from year to year *and more cleverly, as Lord Jim points out in the comments, it's an anagram of DATE'S + YEAR