Is it okay to say "once, on one [name of day of the week]" or does it sound rather unnatural?
For example:
Once, on one Monday, when the homeroom teacher was not in the class, she got on the desk and started signing.
If it's unnatural, than what would be the idiomatic way of expressing some happening in the past that happened at one particular point in time in the past during one particular day of the week?