M.W.
Do not, do not, do not wake him up to say goodbye!
At 20 mos - he really is old enough to understand a little bit that you'll be gone. Prep him the day before that daddy will be home tomorrow with him and answer any questions he has very positively (if you sound concerned - he will be more likely to have a problem). Just be matter of fact and positive - Mommy is going out tomorrow - but daddy will get you up and you'll have a great day (or maybe dad can plan something special and you can use that to prep your son with some as well). When he goes to bed (if he'll be ok with it - if it will make nighttime routine a nightmare - don't mention it again after you've already told him that day) mention again that daddy will get him up and spend the day with him and mommy will see him ..... after nap, at dinner, before bed, up the next day - whenever it is - give him the time when you'll be back so he knows and make sure your husband uses the same terminology with him.
He will have his moments with his father during that day - he may even cry (your husband may also... LOL). No matter who cries - they will be fine and have a great time! I used to feel that way about leaving my husband with my dgtr (and I do work!!!) but have gotten used to the fact that he's a great dad. She may watch a little too much TV - she may get chips instead of fruit for snack, her nap may be at a different time - but she'll have fun and he appreciates what I do more!!!!!
Have a great day - they will be fine and will work it out together. To set your mind at ease a little (if your husband doesn't know the routine - this is what I would do with mine b/c he rarely had to do the whole day thing) write out the routine so he has something to reference. It may help him too. Your husband knows you are leaving - if he gets too little sleep - that's his choice - not your worry! Have fun!