Everyone knows I won't be here on wednesdays, so we try to arrange things so I'm not a bottleneck during that time. If some really important deadline shows up, I'll just work the one wednesday and take thursday off instead, it's not a big deal. I haven't needed to yet though.
Besides, assuming you're not on a really tiny team, if many parts of your project hinge on one person with no available backup, you have bigger problems related to project management.
Of course if you have five people in a team and everybody has a different day off, you can never get together, and that may be a problem. This is the same if you and your colleagues work many days/hours out of the office or at home.
Our dev process required a lot of meetings and interaction. So in order to ensure clear blocks of working time, we used to set aside days as "no meeting days" Some people would take the opportunity to work from home on those days since it was guaranteed they wouldn't be pulled into a meeting. Once you know Sally, Jared and Kaitlyn won't be in on Tuesdays, you plan around it.
It's really not a big deal.
Seriously though, no man should be irreplaceable because of his knowledge, this is some serious risk you are taking.