Christ's resurrection is linked to "Good" Friday. Good Friday is the day during Passover when Christ was crucified. Passover is timed by the Jewish calendar, which is based on the phases of the moon, thus changes every year by a few days.
There is a plethora of Web sites dealing with the timing and date/day of the week of Christ's Crucifixion.
The version of this timing which I have heard before is at this location:
Correlation between Passover and Jesus' crucifixion
Another reference which generally agrees with the first one is here:
Biblical Calendar
So in 2013, although we observed Easter on March 31st, the actual Resurrection per the Jewish calendar places it on April 28th.
So no, we aren't observing the actual anniversaries of Christ's Crucifixion, Christ's Resurrection, His birthday, or any of the other days recorded in His lifetime and the many years before, because we aren't observing the Hebrew calendar. The dates assigned for these observances are based on (1) the Gregorian calendar and (2) a desire to have such observances occur close to similar pagan dates in order to make Christianity more appealing, thus increasing the chances of pagan people accepting and embracing Christianity.
The fact that we remember and revere the events so important to Christians is more important to many than being accurate with regard to the calendar. Since time is much more important to mortal people than to immortal and eternal God, that makes sense.
Thus taking offense or squabbling over these things is as silly as arguing about whether the sun rises and sets or the earth rotates and makes it seem like the sun moves. The effect is the same, so the details are less important in some contexts than the events themselves.