When you first look at the title of this poem, you might be all like, "Great, a poem about Easter." But you won't get very far before you discover that there are no bunnies or colored eggs to be found in these lines. Nope, this poem's about people getting executed for trying to overthrow the British government in Ireland. Not exactly family fare.
Now Yeats could have avoided this confusion and called this poem "The Easter Uprising." But he chose "Easter, 1916" probably for a couple of reasons. First off, he seems to be addressing this poem mostly to Irish people, so just saying the date gives the poem a sort of "insider" quality, because this date would immediately mean much more to Irish folks than people from other countries.
Next off, Yeats' choice to call the poem by a date helps him put the Irish Easter Uprising into a larger historical perspective. There have been lots and lots of bloody battles fought throughout history, and the Irish Uprising on Easter, 1916 was just one of them. This aspect of the title helps Yeats convey the idea that while the Uprising was important, it definitely wasn't one of a kind.
And finally, it reminds us of a running theme in the poem—sacrifice. After all, according to Christian doctrine, Jesus sacrificed himself for humanity's sins, and then was resurrected on Easter. Could that complicated even more Yeats's already muddled feelings toward the Irish leaders of the uprising? Definitely.