According to customs of Vietnamese, the New Moon and Full Moon are 2 of the worshiping ancestor days. In lunar calendar, New Moon is the first day of month (1st), Full Moon is middle day of month (15th). In 2 that days, Vietnamese usaully prepare flower and fruit on altar,  then they burn ancense to worship their ancestor.

   Old woman always remember those days, becuase they are respondsible for prepare offering.Some year ago, I usually go to market to buy food in morning of every day. When I saw every old woman bought flower and fruit, I'm sure that day is New Moon or Full Moon day. Now, I only go to supermarket on weekend and buy food, so I don't see the scene every old woman buy offering on that days.