This is a question you should ask your orthodontist. In general, not much tooth movement will occur if you don't wear your retainer for 2 days, but it really depends on the mobility of your teeth and if your appliance is only to retain tooth position or for tooth movement.
I hope this info helps.
Ideally it is best to wear your retainer each night or as the doctor who made it advised. However, there are plenty of times where people go away for the weekend and forget to bring their retainer.
The first thing to do is to try your retainer back in. Usually there is not too much movement of teeth in 2 days. If the retainer goes in fully, then there is no worry. The retainer may fit a little tight at first but this usually goes away over time. If the retainer does not go in, then it would be best to see the doctor who made the retainer for you. Hope this helps.
My best to you!
William F. Scott IV, DMD