In most states a school year is 180 days long, meaning that the 90th school day is halfway day! I have never been at a school that celebrated that but ran across some mentions of it while researching 100th day options. I thought it was such a fun idea for exploring fractions!
For pre K and kindergarten here is the great Halfway Day article from Scholastic.
For first grade a fabulous set of 5 standards based, interactive complete math lessons (including vocabulary, objectives, accommodations, directions, and assessment) that teach the meaning of one whole, one half, and one fourth. The lessons are engaging, hands on, and enough for the week before and including the 90th Day, Halfway Day
First Grade Kate has a great blog post on introducing these fractions with play dough.
Elementary Matters offers a free TPT download for Half Way Day for 1st-3rd graders including a badge to decorate and wear and two practice pages for learning about halves.
Books you could read to your young students include My Half Day and Give Me Half.
Upper Elementary Grades
In upper elementary grades it would be a great day to explore symmetry. Build symmetrical designs with pattern blocks. Give the kids halves of drawings and have them draw the other half. Teach them to make symmetrical hearts or butterflies by folding and cutting paper, and show them how the halves will be reflections across the fold line. It would also be fun to teach the kids to make paper snowflakes. Great books for exploring symmetry are Seeing Symmetry (aligned with 4th grade Common Core geometry standards) and What Is Symmetry in Nature?
Upper elementary could also practice equivalent fractions. 1/2=9/18=90/180=900/1800. Keep building the pattern; they love creating bigger and bigger numbers!