On this day we recognize the anniversary of Lama Tsongkhapa’s (1357-1419) parinirvana. The date follows the Lunar calendar, meaning that it does not occur on a set date in the way that Christmas occur every December 25th.
To celebrate Lama Tsongkhapa Day, you can perform various Dharma practices, such as light offerings that symbolize the light of the wisdom of the Dharma using your Christmas tree lights. But the main thing is to practice with sincerity and a positive motivation to benefit all sentient beings. Any practice that you can do today would be awesome.