Thanks to KB for this information about Pi Day:
*Pi Day is celebrated on March 14th (3/14) around the world. Pi (Greek letter “π”) is the symbol used in mathematics to represent a constant — the ratio of the circumference of a circle to its diameter — which is approximately 3.14159. Pi Day is an annual opportunity for math enthusiasts to recite the infinite digits of Pi, talk to their friends about math, and to eat Pie.*
Here are some thoughts on Pi pies – – –
and some from Getty (from 2016) – – –
But it does not have to be all about food (though that is a pretty good way) – – –
here is a pi-day site – with lots of ideas – many for teachers and others  –
Note: it is also the birthday of Albert Einstein, so a cake would also work.