How to compress multiple folders into multiple zip files
In your case you can combine two loops to accomplish this
cd YEAR
for month in */; do
    (cd &quot;$month&quot;;  for i in */; do zip -r &quot;${i%/}.zip&quot; &quot;$i&quot;; done)
done

PS: If MONTH folders can be empty you need to check for this in the inner loop.