Dec. is simply the short written form of December, the twelfth month. You see it on calendars, letters, forms, and dates: Dec. 25, or 25 Dec. 2026. In American writing the full stop after Dec. is common; British writing often keeps it too, because letters really were cut. Say 'December' when you speak — Dec. is for the page, not for the mouth. Though actually, people sometimes write Dec without the dot in messages and on phones, and nobody gets confused.

十二月（寫作用的縮寫）