at one o’clock, at three thirty, …

at lunchtime, at breakfast, at dinner, …

at noon, at midnight, … but in the afternoon, in the morning, …

at Easter, at New Year, at Christmas, … but on Christmas Day, on Easter Sunday, on my birthday, on New Year’s Eve, …

in the evening, … but on Saturday evening, on Friday afternoon, on Sunday morning…

on Saturday, on Monday, on Tuesday, …

on the third of March, on July the 21th, …

in June, in 1997,

in spring, in the winter, …

(Br) at the weekend / on the weekend