This is deceptively complicated, because Send Later works by periodically checking whether there are messages that need to be sent. If it finds one that's past due, then the default behavior is to send it immediately. You can prevent that by setting the "Don't send messages more then ____ minutes late" option in the Send Later preferences, but then the message won't be re-enabled until you schedule it again manually. There isn't currently any "skip the next recurrence" sort of functionality. Depending on how important this is to you, the one option available at
the moment would be to write a custom scheduling function, assuming you
know ahead of time which days you'll want to skip.
The short answer is that there isn't an easy solution to this. You'll
likely have to re-schedule the message by hand when you want to skip a
day.