I have seen this in a few places online. I know it's a celebration, but what is the significance of it?

It's basically a way of incorporating number sense into the classroom in a fun way... many primary classrooms keep track of how many days they've attended school, and it helps them learn the concept of one hundred.

On our way to 100, we learn place value, skip counting, money, etc. Then the 100th day is a celebration that ends up being a lot of work for the students, because any activity you do with the number 100 takes a long time! You usually see 100th Day celebrations and activities more often in primary classes, although I'm sure some teachers of older grades have some creative approaches.

It's just a fun way of learning the number 100!! We count by ten's and by 5's, make 100 day projects, etc. I make a big deal telling the kids that they are 100 days smarter...

I just took a math workshop that stressed that teachers don't stop at 100 but carry on for the whole year. Students have a difficult time gettting from 100 and then 1000. They need to see how to continue counting or they go 109 to 1010.