sumer, m.n: summer. (“suh-mehr”) May 9 is the 1st day of summer in Anglo-Saxon England.
Swylce ymb fyrst wucan
butan anre niht ϸætte yldum bringð
sigelbeorhte dagas sumor to tune,
—The Old English Metrical Calendar (Menologium), lines 87b-90a
Likewise it is after a week but a night that summer brings to town among people sun-bright days and warm weathers. (trans. by K. Karasawa)
Karasawa says that the beginning of summer, as stated here, is six days after the Invention of the True Cross (see bēam). Apparently many Anglo-Saxon calendars specified May 9 as the beginning of summer.*
*Kazutomo Karasawa, The Old English Metrical Calendar (Menologium) (Cambridge, 2015), p. 103.