1. "At" is used for time and certain expressions: - at 8.00 - at lunchtime - at Christmas - at the moment - at the weekend - at the same time - at the age of 5 2. "On" is used for days and dates: - on Friday - on 12 September - on Friday morning 3. "In" - for longer periods such as months, years, seasons, etc.: - in January - in 2012 - in summer - in the 20th century

'Christmas Eve' is a day (24 December) . We always use ON with days, so you say 'on Christmas Eve.' It's the same use of 'on' as when we say 'on Christmas Day'. The word 'Christmas' on its own refers to a festival, so we say 'at Christmas', the same as 'at Easter' or 'at Ramadan'.

it's " ON " :)