Sol-mōnaþ, m.n: the old name for February. The “sol” part is of doubtful meaning, “mōnaþ” means “month”. (“sol-mon-ath”)
Further information from Kazutomo Karasawa’s The Old English Metrical Calendar (Menologium) (Cambridge, 2015)
Swylce emb feower wucan
ϸætte Solmonað sigeð to tune
butan twam nihtum, swa hit getealdon geo,
Februarius fær, frode gesiϸas,
—The Old English Metrical Calendar (Menologium), lines 15b-19a
Likewise it is after four weeks but two nights that Solmonað advances to town, as the wise men, the ancient people learned in the laws (of reckoning) reckoned it, the coming of February. (trans. by K. Karasawa)
Karasawa says of Solmonað, ‘Its etymology is not certain, but its first element has often been connected either with Old English sōl “sun” or sol “mire, mud”’ (p. 89). According to Bede’s De temporum ratione, ‘Solmonað can be called the month of cakes which they (pre-Christian Anglo-Saxons) used to offer to their gods’. Karasawa observes that Old Norse sólmánuðr ‘sun month’ is the third month of summer, so presumably not February!
For more on this etymology see K. Schneider, “The OE Names of the Months”, in Sophia Lectures on Beowulf, ed. S. Watanabe and N. Tsuchiya (Tokyo, 1986), pp. 260-75.