I also don't pronounce the T, but I wouldn't call it a silent letter because it can be pronounced. Some do and some don't. It's a question of usage. Brits are more likely to pronounce the T than Americans. It's similar to the T in "soften" - you can say it either way.

Californians, typically, do not pronounce the "h" or the "t" in Christmas. With the California accent, it sounds like "kris-muhs." However, I have heard Americans from other regions/states pronounce both syllables, and I have heard variations where one or the other syllable was pronounced. For example, some southerners pronounce it "kurah-ist-mahs." All pronunciations are essentially correct; it boils down to a difference in accent. I'm sure that is true in every country.

Christmas = kri-smuhs