Updated the tengwar to match Tolkien's letter.
nai lye hiruva airea amanar!
The letter was discovered in an auction on July 10th, 2019. The letter contains an ancient Elvish translation of 'Merry Christmas'.
You might want to provide the original gloss as an alternate translation: “may thee find a blessed Amanar”.
Elaran and I discussed it on Discord and agreed that Amanar is probably am “up” + Anar “Sun”
Yes, it must be referring to the winter solstice, after which the "Sun" starts going back "up" (having reached its lowest point in the sky).