Does it matter what day of the week I choose to open and close my cart?

I recommend scheduling cart open/close on a Tuesday, Wednesday, or Thursday, but you know your audience better than I do. The reason I recommend these days is that the middle of the week tends to be the time most people are online. 

Mondays are not the best days to market anything. People are just getting back into the swing of things and have not necessarily caught up on their email. Friday is also not an optimal day for open/close, since many people leave work early, go on vacations and are generally less engaged. (Weekends also are not ideal, for the same reason.) On Tuesday, Wednesday and Thursday, people are more in tune and are paying closer attention to email. Do what feels right for you and your business, and you can always adjust for your next launch.