The period of time after Christmas and New Years where the clothes you wore prior to the holidays that fit well, all of a sudden are a little tighter than they should be, which is called “Christmas tight”.
“Christmas tight” happens because there is so much delicious food during the big holidays, and you just cannot help yourselves. Therefore you put on weight in a few days.
"I got this new dress shirt for Christmas. I went to wear it with my favorite work pants, but they are a little Christmas tight now."