When we ask "When is Christmas?" we mean what day of the week it falls on. When we ask "When is Easter?" we mean a specific numeric day in March (never before the 22nd) or April (never after the 25th).
Easter Sunday is key, of course, to our common life and to our calendars, with Ash Wednesday, Pentecost, and several other important days being set by its placement. But if you've ever tried to work your way through the Prayer Book's rules for finding the date of Easter Day, you know how complicated it can be.
How'd it get that way? Bosco Peters opines,
The dating of Easter arises from the complicated joining of two different calendar systems. These calendars might be illustrated by the early story of Cain and Abel. If you are an Abel type – hunting, fishing, watching your flock by night, you will focus on the moon and the lunar cycle of 29 and a half days. Moonlight and tides will be significant to you. If you are a Cain type, a tiller of the ground and grower of crops, the solar cycle and its seasons will be more significant to you. The dating of Easter comes out of combining these solar and lunar calendars.