The Lunar Calendar

This year I noticed something that I didn't last year -- Vietnamese have two calendars in their heads. I always knew that in Vietnam the lunar calendar was important but I didn't realize people actually always knew what day it was on both calendars. Maybe this just happens around Tet but I kept hearing people say, "Today is the X" when it was clearly Y. Finally I saw a calendar on a wall and noticed that both Western and lunar dates were on it. I've seen these calendars many times but never before noticed the dual dates:

(PICTURE: The big date is for the Western calendar and the small date is for the lunar calendar.)