Use on in and at when talking about time.
ON→ days and dates
I hate to get out of bed ON Monday’s.
IN → Months, years, seasons and part of the days.
He loves to read IN the evening.
AT → Time (o’ clock, speciale dagen), plekken
My alarm went off this morning at 6 O’clock.
I met Chris at the movies.
We hid our presents at Christmas/ easter/ his birthday.