We use "下周" to express "next week ".
We use "下个月" to express "next month ".
We use "明年" to express " next year".
"周、年” can be used as the classifiers, so you can say "下周”,“明年”.
While "月" only can be used as a noun, so we usually say "下个月".
Besides , you have to consider the usage .
For example , in English , we usually say "yesterday afternoon " and "last night ".
When we say it in Chinese, we usually say “昨天下午”and “昨天晚上”。