Almost everything that can be said with confidence about Eostre rests on one sentence. In his work On the Reckoning of Time (De Temporum Ratione, 725), the Anglo-Saxon monk Bede, explaining the old English month-names, writes that the month answering to April was called Eosturmonath, formerly named after a goddess of the Anglo-Saxons called Eostre, in whose honour feasts were held in that season; the name, he says, was afterwards kept for the Christian Paschal (Easter) month. That is the whole of the ancient evidence.
No myth, no image, no cult-site, no second early witness survives to tell us who Eostre was, what was done at her feasts, or how widely she was worshipped. Everything richer than Bede's brief remark is later reconstruction, and it is important to be clear that the picture of a great, well-known pagan spring festival is not something the sources actually supply.