Eastertide
noun
Eas·ter·tide ˈē-stər-ˌtīd 
: the period from Easter to Ascension Day, to Whitsunday, or to Trinity Sunday
Word History
Etymology
Middle English estertide, from Old English ēastortīd, from ēastor + tīd time — more at tide entry 3
First Known Use
before the 12th century, in the meaning defined above