at: a time ( exp: at six o'clock, at midday) /
a mealtime ( exp: at breakfast)
on: a day (exp: on friday, on my birthday) /
a date (exp: on september, on the sixth of May)
In: a part of a day (exp: in the morning) /
a season (exp: in the spring) /
a month, year or century (exp: in July, in 1990)
