You should say “月底”.
"底" means the bottom end. For example "鞋底" means sole of a shoe.
"低" means low.
2017年2月15日
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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
2017年2月15日
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Thanks to all for the detailed answers, they are all helpful.