Sunday begins at sundown on Saturday evening. That is why the LOTH for Sunday there is no Saturday evening prayer, we begin with Sunday evening prayer I.
The Easter Vigil may not begin until after sundown on Saturday, April 15 this year, and should conclude before dawn. It begins after it is completely dark because we begin with lighting the Paschal fire, and to be an effective sign it should be dark, and the church should be dark. Depending on how many people are being baptised it should take 2 to 2 1/2 hours. Eastern Rite and Orthodox service is much longer, almost all night long.