Notice the use of the prepositions of time in and on in these common expressions:
in on
in the morning on Tuesday morning
in the mornings on Saturday mornings
in the afternoon(s) on Sunday afternoon(s)
in the evening(s) on Monday evening(s)
When we say last, next, every, this we do not also use at, in, on.
• I went to London last June. (not in last June)
• He's coming back next Tuesday. (not on next Tuesday)
• We'll call you this evening. (not in this evening)

(Adapted from Englishclub.com)