Answer:
It is often rainy IN autumn.
Step-by-step explanation:
IN:
Streets
Specific places
Cities
Countries
Continents
Parts of a country / region / city
Number of weeks / months / years
Parts of the day
Months
Seasons
Years
Decades
Centuries
ON:
Weekdays
Special days with the word "day"
Specific dates
AT:
Specific places in towns or cities
Hours
Parties and festivals without the word "day"