AP Style holds that you should not use “on” before a date or day of the week when its absence would not lead to confusion, except at the beginning of a sentence. For example,
- The conference call will be held Monday.
- The president will be inaugurated Jan. 20.
- On Dec. 21, the committee will meet to discuss the issue.
Use “on” to avoid an awkward juxtaposition of a date and a proper name. For example,
- Steve met Susie on Monday.
- She told Johnson on Thursday that the appointment was cancelled.
Use “on” also to avoid any suggestion that a date is the object of a transitive verb. The example,
- The Senate killed on Tuesday a bid to raise the debt ceiling.
- The House postponed on Thursday budget talks