Well, it’s April 16 this year. But Easter is one of those holidays that falls on a different date each year, based on a formula that dates back to 325 A.D., when the Council of Nicea laid down ground rules based on the Gregorian Calendar. The formula was adjusted slightly in 1582 to account for leap years. The variables include lunar cycles, and are explained in detail by Time.com, while astronomers provide the dates for Easter for many years to come. Easter always falls between March 22 and April 25.
In the Western church, that is. Orthodox Christians follow a slightly different formula for setting the date of Easter, explained at Time and Date and in some detail by The Greek Reporter. The difference dates back to the changes in 1582, at which point the Eastern church had already separated from Rome. In most years, Orthodox Christians observe Easter a week or more later than the “Western” church. In 2017 Orthodox churches also observe Easter on April 16.