When considering a set of days and writing about them in a 'diary mode', what preposition should I use in the following example:
- He died at day 5 of the treatment.
- He died on day 5 of the treatment.
"On" works better here. It implies that he died at some point on the 5th day of the treatment. I would use "at" to reference a very specific point in time (rather than a relatively longer period of time, such as a day), for example: "He died on the 5th day of the treatment at 3:02 PM".