We just finished Kiddush Hachodesh, which I find fascinating. First of all it's fascinating that people get so caught up in the science of these perakim, when the Rambam seems to be pretty clear that all that matters is the math!
And now that we started Taaniyos, I thought it was very interesting that as awful as coronavirus has been, it still doesn't qualify as a plague requiring a communal fast according to the Rambam! (The ratio for a plague is 3/500 men per day for 3 consecutive days.)