Alas, WttH/TH have to happen in January. There's no way for Witch (much less Halloween Rain) to occur in 1996, despite filming much of the season then and Witch being the first episode filmed. Most of Angel's episodes (along with scenes mentioning him dropped into later drafts of early scripts) are notably some of the last dated scripts, as if they realized late into scripting that they wanted him way more prominent (we know it was the WB who wanted that and Joss had to be talked into it by other writers--Greenwalt got the task of creating the backstory and the whole soul/curse mythology). Outside of the opener, those episodes are the ones heavily filmed in early 1997, not 1996.
Giles says this in The Harvest: "'On the night of the crescent moon, the first past the solstice, it will come.' Of course. Tonight."
Solstices only occur in June and December. The first week back to school in January 1997 is indeed during a crescent moon phase (multiple nights, though it went new moon on January 8, which means it's at a sliver the night before during the Harvest) and the first after the solstice on December 21, 1996.
Also, Teacher's Pet explicitly has the replacement teacher talking about late-January mid-terms.
Though I sincerely doubt they expected anyone to check moon phases! It certainly helps date every episode/book Oz/Nina werewolf event.
