So I accepted an offer and need to give my two weeks notice, the thing is my boss' last day is tomorrow. What are the pros and cons of telling him tomorrow or waiting until next Tuesday (we are closed Friday-Monday)?
Short answer: You should inform your current employer as soon as is practicable.
If you know you are leaving the organisation (with a set start date for your new role), then your boss's departure date is largely irrelevant. You should inform them as soon as you can, so that they can prepare looking for a replacement and so you can perform a directed handover of anything you have been working on.
From your comments, you are worried about the effect on the team's morale. Honestly, it won't make any difference whether you do it tomorrow or Tuesday from that perspective.
Your boss's departure should not matter for you. You should hand in your resignation in a timely manner. It is in your interest to make sure no periods are violated and in the employers interest so they can plan accordingly.
However, when you hand it in the day your boss leaves, it is possible that he doesn't find time to process it. Your resignation could then get misplaced during the responsibility transfer to the new boss. His replacement will need some time to get into their new job and might simply miss it. So I would recommend you to CC it to the next higher person in the company hierarchy and to the new boss (when you know who they are) to make sure it gets processed.
Inform your employers straight away - it's nothing personal, it's just business.
The way I approach questions like this in my career is to think "how would the company act if the shoe was on the other foot". For example, if they planned to lay you off to cut costs would they do it tomorrow or decide "lets wait until the new guy is in and then have him do it".
Sounds like a sinking ship.
Obviously there must be something wrong with the company if everyone is leaving simultaneously.
Don't worry about the collateral damage, they brought it on themselves. Worry about #1 first. #1 is you.
Tell your boss ASAP, he can do the dirty work for you and submit it to HR. It's no problem for him, business as usual since he's leaving anyways.