Prepositions of time: 'in'

in

We usually use in with parts of the day and longer periods of time such as months, seasons and years.

I usually relax in the evening.
In summer it's too hot to do anything.
I'm always really busy in December.

parts of the day

in the morning/afternoon/evening

months

in January/February

seasons

in (the) spring/summer/autumn/winter

years, centuries, decades

in 2016

in the 21st century

in the 80s