Does anyone know at which temperature ponds freeze enough to allow sims to skate on them? For some reason, in one of my saves, whenever sims skate (whether in either rink or a pond), the game freezes.

It's not my mods. I only have one affecting skating, and the freezing occurred both before and after.
It's not the town (I think), since skating worked in a fresh copy.

I wouldn't be so quick to dismiss the mods possibility without checking what happens now on a test run with no mods in play. A mod that affects routing could do this. It's a known issue for some players with the version of NRaas GoHere currently in beta testing, for example (that would be v44, not sure where the issue crept in but it's reportedly still there in 44g).

Sorry to hijack, but this reminds me of something odd involving skating that I saw, having returned to TS3 from TS4 recently. When I had Seasons and Late Night installed, a teenage vampire in my household would run around the ice skating rink. I don't recall if I ever sent him roller skating to see whether the same thing would happen. Other non-occult Sims in my household were able to ice skate or roller skate without incident.

When I played TS3 regularly a few years ago, I remember being of the opinion that certain of the expansion packs didn't play well with certain others (and that Island Paradise and World Adventures didn't play well period, but that's another story). Mods are always an issue, though it seems for you the problem disappeared with a fresh game (I assume with the same mods installed). In the end in my case, I attributed it to something wrong with that particular Sim, possibly unrelated to his being a vampire, as the game slowly inched its way closer to becoming the inevitable Big Fiery Ball Visible From Space.

Quite a lot of supernaturals are not able to skate. Witches can skate and learn the skill but cannot make use of the skill perks like pirouettes and backwards skating (inactive witches, however, can for whatever reason), genies will float, vampires will run, fairies will fly, mummies will shamble as usual and werewolves can only skate when they are not transformed and neither of them will gain skill points in skating, IIRC.

There were attempts at NRaas to unlock skating for supernaturals but I guess they haven't been successful yet.

Occults in particular not being able to skate properly is a "feature" of the game itself. Twallan tried to unscramble that one just before his retirement with a version of the NRaas Hybrid mod that never quite made it to beta testing. Unfortunately that one broke more than it fixed. The one currently in beta testing, ver 10c, seems to be doing a better job of it.
http://nraas.wikispaces.com/Hybrid+Testing

I do have GoHere in the mods folder. Version 44d. I'll check other saves to see what happens. I don't have sims skate much. Inactives are really the only ones who do on the frozen ponds. Normally I don't care. I do this time because it's causing the game to freeze in a world that's in week 9.
In the meantime, I might just put a fence around my festival ground's pond.

This is a rough trade-off until we get that beta version of GoHere fixed. Like you, I can work around not having my actives skate for a while but can't deal with my game crashing or locking up every time an inactive tries to either. I'm on 44g and would really not want to lose all of the intricate door locking and charging for entry structures I've since put into place, nor its ban on boat routing that does work well for me. There was a 44h briefly, but some of us never even got to test that one as it was a bust and was sent back to the lab for more work.