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”。