Using this more, my view is always that "at" need to be used for activities that aren't times on the 7 days (at Xmas, at Easter, within the weekend, at lunchtime, at nine o'clock) irrelevant of size, and "on" where by the time is on a daily basis of the 7 days (on Saturday). I would argue this is to carry out with the fact that "at" indicates a cer