We use 'in' to talk about:
- Months / Seasons: in April, in (the) summer, in (the) spring;
- Years / Decade / Centuries: in 2020, in the 70s, in the 21th century;
- Long periods: in the present, in the past, in the future, in the ice (middle) age(s);
- Parts of the day: in the morning, in the afternoon, in the evening.