In modern Chinese, 圣诞 (Shèngdàn) almost always means 'Christmas' — you'll see it in greetings like 圣诞快乐 (Merry Christmas) and compound words like 圣诞老人 (Santa Claus) and 圣诞树 (Christmas tree). Historically, the term literally means 'holy birth' and was used for the birthdays of sages, Buddhas, or emperors, but that usage is now very rare outside of classical texts. For everyday conversation, just think of it as 'Christmas'.