Christmas tight refers to the period of time after Christmas and New Year where the clothes you wore prior to the holidays that fit well, all of a sudden are a little tighter than they should be.
Christmas tight happens because there is so much delicious food during the big holidays, and you just cannot help yourselves.
wǒ wèi shèngdànjié xīn mǎi le zhèjiàn chènshān. zhīqián wǒ yìzhí ná tā hé wǒ zuìài de gōngzhuāngkù dāpèi, dàn xiànzài yǒudiǎner jiéhòu suōshuǐ le.
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.
Source: China Daily