An art gallery that's been around for more than a few months probably has a calendar with exhibitions in the past, present and future. In what order should these be presented on the gallery's website? In general, current exhibits should be first, then upcoming exhibits and lastly past exhibits (hence the mnemonic CUP).
Of course this also applies to artists' websites, as well as websites that aggregate such information from multiple galleries and artists (like this website). It all seems perfectly obvious, but when I started working on this website, I was not consistent about this. You might even be able to find a page on this website for some gallery or artist listing exhibitions in an order other than CUP.
The CUP order is the best order. "Be mindful of the future ... but not at the expense of the moment" is a bit of wisdom from the movie some consider to be the worst Star Wars movie so far. And then there's the common wisdom that "those who don't know history are doomed to repeat it." Hence the CUP order is justified thus:
It's not difficult to find gallery websites using a different order, and maybe there's a good reason for the different order, maybe there isn't. There certainly are more important issues for a gallery's website, like making sure the street address and gallery hours are on every page, or sizing the images so that they both load fast and look good. But if you settle on using CUP, that's one less thing to worry about.
What about when the gallery is between exhibits? Simple, just gray out or hide the current exhibits section. (With this website I do the latter by "commenting out" the section, which you can still see by looking at the HTML source). Similarly, the past exhibitions section can be empty but ready for a gallery that's just starting out.