Ответ:

at: at 8 o’clock, at 11 o’clock, at noon, at 12 o’clock, at the weekend.

on: on Sunday, on Monday, on Wednesday, on Thursday, on May 1st, on the 22nd of June, on the 7th of January, on the 9th of May.

in the: in the morning, in the afternoon, in the evening.

in: in the night, in spring, in May, in summer, in winter, in autumn, in 1945, in June.

Употребляем at с точным временем и выражением at the weekend; on — с днями и датами; in — с месяцами, временами года, годами и частями суток с артиклем the.