Fill a jar (or several) with candy or items relating to that holiday. For example, if it's Halloween you can fill a jar with spider rings or Valentine's with heart erasers or heart shaped candy, etc. Let each student guess how many items are in the jar. The student with the closest guest wins the jar! If you are using this game in a classroom with younger children, it makes a terrific counting game also. Gather the children and let them help separate the candy into piles of 10. Have the entire group count the piles by 10s – educational and yummy!
As a side note, my son's 2nd grade teacher had a sign-up sheet at the start of the school year for parents to take part in this monthly activity. At the start of each month, the teacher sent home the designated estimation jar to fill. The students looked forward to having the estimation jar filled each month and being able to take part in this fun activity.