Why is it "on" rather than "at" here?

Rebecca

Remember that we use "on" in reference to units of time, such as days of the week, when using "days" or "nights," and specific dates. "At" is only used in the context of "during night time" and never in any other situations. Just to be safe, you should always use "on" instead of "at" as situations requiring its use are far greater. Ex: Let's meet for lunch on Monday. Ex: My birthday is on April 1st. Ex: I work out on weekday nights. Ex: I like to sleep early at night. Ex: I can hear wild animals outside my window at night.