Best time of day to deliver daily notifications?

A while ago I wrote an app for myself that automatically checks if I've been near my gym every day. If I miss a day, I get a mean message the next day like "Missed the gym yesterday, you fat fuck". I'm thinking about publishing this app to the public as "Abusive Gym Reminder".

Right now I'm not sure what time of day to send the message notification. For myself, I just set it for 4:00PM, but for a wider audience I would like to time the deliveries so they are relevant but pleasantly spontaneous.

Are there best practices for timing the delivery of daily messages? I have apps (like Google, Dominos) that send me pseudo-regular notifications but I'm not sure how they figure out when. Like, if I made it always around noon +/- some noise, would that be acceptable?