Here is my advice from 5 years ago when my son was going through this:
The decisions for the TSAO schools come out at multiple points throughout the day. The first one I believe was Exeter, who sent out an email at about 5 am. The non-TSAO schools sometimes send them out earlier. A local private day school near us had already sent a postal mail acceptance about a week prior. You might consider asking your child to not check any of the decisions until after he/she comes home from school. Especially in case of a rejection/waitlist, I wouldn’t want my child to be in front of your friends when they open the decision. I’d rather they be home with me.
Several of the TSAO schools will send out admission packets on M9 with the intent of arriving on M10. In the past, some parents have signed up for all of the various arrival notification services (Fedex, UPS, USPS, etc) to get advanced warning. Andover for example sent a Fedex package to our home which was mailed on M9. Some other schools wait until after M10 to send out the formal package.
I would also be prepared for rejection. Get something comforting to your child: favorite candy, food, etc. so that you can offer right away. Definitely would not recommend buying any school apparel until after the acceptance is in hand. Plenty of time to do this during the revisit days.
In terms of how to celebrate? We went out for dinner nearby. I think most of the decisions are out by 5 pm. You could also consider a movie, ice cream, etc.