When is the best time to go to Bay Street Apple store to have a battery replacement? Do I need to make an appointment? Is it same day service? Can I find Apple Genius Bar staff somewhere other than an Apple store?

I would suggest making an appointment i just had my computer fixed and i found it to be pretty straight forward. When you make an appointment it will ask for the reason of your visit whether its repair or something else but it went smooth had my computer back within 2 days.

Check online before you go to the store make sure the battery is in stock. Check-out the hours of operation then get there before they get busy. Just remember Covid-19 restrictions are in place and being enforced.