In elementary school, to mark the 100th day of school, everyone brought in 100 of some item. One year, I brought in miniature oreos, vacuum-sealed by my father when he sucked the air out of a ziploc bag. Another year, I brought in pennies. Other kids brought in matchbox cars or bottle caps or candy that was then shared out to the class.
Looking back--I have no idea what the point was. A math exercise? Proof that we could count that high? A free holiday for the teachers, since we did other silly things together with other classrooms and other grade levels? The 100th day was not even a metric point in the year--our school years were around 180 days, so the 100th day of school generally fell just after school started up again in January.
100 posts, two years+. Huzzah!