I think that Christmas in the summer would be weird. Mainly because, to me, Winter is the end of the year (probably because I hate it so, all the cold and gloom), and because we enter the New Year right after Christmas.
And Christmas (or Hanuka, Rhamadan, Solstice, etc.) are like the Grand Culmination Climax of the year. There is no other holiday that has as much fanfare or whoopla as Christmas - not Easter, not Thanksgiving, not the Fourth of July, and not even, sorry to say, Jerry's birthday.
So it's just fitting to me, that the grandest holiday of the year comes at the end of the year, so we have something to look forward to and build up to, before we start all over again in a fresh new year.