The words summer and winter are both used only once in the OLB.
I can imagine that the oldfrisian word for summer, SÜMER, is related to the name of the ancient civilization in what is now Iraq.
summer - english
sommer - danish, german, norwegian1
simmer - frisian
sommar - swedish, norwegian2
sumar - icelandic, oldnorse, oldsaxon
sumer, somer - oldfrisian
sumor - oldenglish
zomer - dutch
SÜMER - OLB
winter - english, dutch, german, frisian, oldfrisian, oldenglish
vinter - danish, norwegian, swedish
wintar - oldsaxon
vetur - icelandic
vetr - oldnorse
WINTER - OLB
OLB fragments
[049/12] arge tid
HÉL THENE SÜMER WAS SVNNE ÀFTERE WOLKUM SKOLEN
AS WILDE HJA JRTHA NAVT NE SJA
[O+S p.71]
Geheel den zomer had de zon achter de wolken gescholen,
als wilde zij de aarde niet zien.
During the whole summer the sun had been hid behind the clouds,
as if unwilling to look upon the earth.
[082/04] denamarka
THES NACHTIS ANDA WINTER BY STORNE TÍDUM
AS WIND GULDE ÀND HÉJEL TOJENST THA ANDÉRNA FÉTERE
[O+S p.113]
bij nacht in den winter bij stormweder,
terwijl de wind gierde en de hagel tegen de vensters kletterde.
one stormy winter night,
while the wind roared and the hail rattled against the windows