'Oestre' and 'Easter' probabably sound similar. And later on, 'Easter' was adopted in the use of English.
The Easter symbolizes of the Jesus Christ's return to life after he was killed. And 'Roman Oestre' is the goddess of reproduction, which has quite similar meaning as 'Easter'.
I think when christianism appeared, and many people wanted to make it The religion, they "converted" some pagan celebrations, or at least kept the same dates or the same basis, because quite a lot of people were attached to them. Thus the spreading of the new religion went more smoothly.