You should say “月底”. "底" means the bottom end. For example "鞋底" means sole of a shoe. "低" means low.

月底。 "底"means something's “bottom”, it;s a noun, so at the end of the month, we use "月底" "低"means “low” it's an adjective. e.g. the water level is very low-水位很低

Thanks to all for the detailed answers, they are all helpful.

底， 低 is low 。short ， but 底is end 、 bottom

you should say"月底“means at the end of one month.