If it happens frequently, you may want to question your desire to get in the gym, or the practicality of being on a strict schedule.
For me, I work three jobs, one of them til 4am 6 days a week. The gym I'm at isn't 24/7 anymore. So I said f#$k it. I lift when I can get in there, which tends to be 2-4 days a week, on no set schedule. Before I started logging my days, this led to not getting into the gym as much as I would like, so if you go this route, be sure to keep track, and make sure you don't go two weeks without a lower body day, or some such nonsense.
If it doesn't happen frequently, just here and there: not a big deal. do your Weds. workout on Thurs, and try to come in Fri or Sat to get in another one. If you have to go lighter, it won't kill ya. Also a day off won't kill you.