In our Skype English classes, many students say ‘in Halloween’ but we must use the preposition of time ‘on’ for single days such as Halloween, all saints day, Easter Monday, New Year’s Eve or individual days such as the 13th of March or Friday. So for instance we could say ‘many children knock on my door on Halloween’ or ‘on Friday I will go to a fireworks display’. Remember, ‘on’ is for single or individual days. Please do not use ‘in’ for individual days. As a time preposition ‘in’ is used with months (e.g. March), seasons (winter…) and years (1978…). Please feel free to ask about this if you have any doubts. Thanks!