This list is from some website that had the airdates every week for every
Star Trek show, TOS, TNG, DS9, VOY, ENT. I don't know where the site is unfortunately (I copied the actual text a few years ago, but that's on another computer). There's a comprehensive archive somewhere for The Twilight Zone too, including what those pre-emptions were (it was pre-empted in the 1960 summer for political convention coverage).
Something I'm curious about but it seems no one has done is look at what the original schedule was for those few days that everything was pre-empted due to the JFK assassination. It cancelled everything on Friday, Saturday, Sunday, and I think Monday too. And it shuffled airdates too- everything wasn't postponed by 1 week. The Twilight Zone episode intended to air 11/22/63, "Night Call", aired in Feb 1964, for example. There are rumors the Route 66 ep "I'm Here to Kill a King" was intended to air on 11/22/63, though I haven't been able to verify that (for those who haven't seen it, umm... while it's set at Niagara Falls and involves some arab leader and a twist that didn't occur in the real event, it involves a philosophical wanderer using a rifle to shoot from a grassy hill at around noon). That episode was postponed, airing after the series finale in 1964. I forget which Outer Limits episode was intended for 11/25/63. I think it was "Tourist Attraction" but I can't remember exactly. Someone has to have kept a tv guide from that week somewhere, but I'm not sure if the info in it is online.
That's not uncommon. I can give you numerous examples of TV shows where an episode filmed early or in the middle of the season, which was considered to be an inferior or poor episode, was held off for airing till the end of the season when ratings don't matter.
Yes, this was particularly common back in the 50s/60s. Look at Leave It to Beaver production numbers and their air order. In May or June, you'll see some episodes filmed early in the season air there. In fact, the 2nd to last episode is one of the series' worst episodes. It was filmed at the beginning of the last season. It got less common as season size shrank, meaning there's less of a stock of episodes to pick from if a show thinks an ep is weak.
There are some episodes that get barely salvaged, having a lot of behind the scenes stuff we never really learn about. I know the X-Files episode "Detour" was a trainwreck, taking 19 days to film and being waterlogged for much of the time, they needed to recreate some wooded scenes in a studio and film it there. Of course, X-Files always barely made their airdate, and "Gethsemane" even had differences between the East Coast feed and the Pacific Coast feed of the original broadcast. And there are things that get scrubbed from memory (DS9 "Rules of Aquisition" [no c], and VOY "Basics" having 2 versions circulating around, "Basics" and "Basics, Part l"- VOY always favored "Title" & "Title, Part ll", never "Title, Part l", though they used "Basics, Part l" for the original broadcast in 1996), and the Babylon 5 season 1 finale having a missing effects shot in its original broadcast (Londo walking around some hedges in a small studio instead of in a giant outdoor hedge maze). I know other shows had behind the scenes problems in producing episodes like "The Alternative Factor", with recasting or quickly doing major changes to the script though I'm drawing a blank which series/episodes though.
! Xena. In Season 2, you'll notice a strange episode where it's Gabrielle & the villain Callisto, but she's acting like Xena. Lucy Lawless had a horse riding accident and broke something, so they quickly modified the ending of another episode (aired back to back but it was produced several eps earlier) so that Xena & Callisto switched bodies for a time and just had the Callisto actress take Xena's part with a few minor dialogue changes and 1 new scene on the end when Lucy Lawless could return to the show. Hercules had something similar. Kevin Sorbo had a shoulder aneurysm in Season 4 so couldn't do much lifting or heavy work, so the supporting cast became the stars of several episodes or they used actors from an associated series (Young Hercules) to film an episode or two. It had a lot less action compared to other seasons and a lot more comedies and episodes with a lot of talking compared to other seasons.
Voyager has a small trace like this, 4 Season 1 episodes being held over for Season 2 and 4 Season 2 episodes held over for Season 3, with Season 3 only having 22 episodes produced (instead of the standard 26). The reason seems to be Paramount thought the 4 episodes would air in June, like how DS9 & TNG aired, but UPN refused to air episodes after the final May sweeps week, thus they were held back. This created a backlog, so instead Season 2 being 30 episodes, they offset 4 Season 2 eps and cut 4 eps from Season 3 (UPN perhaps being cheap and not wanting to pay for 4 "extra" episodes). I have no clue why Voyager Season 5 premiered so late (mid Oct 1998) though. The surrounding premieres were around the beginning of Sept (95, 96, 97), or mid Sept (99), with the 2000-1 season being postponed to early Oct because of avoiding competition with the 2000 Olympics in Sydney. Voyager aired 3 episodes in December 1998 (must be a record for Star Trek since TOS). I tried to dig around but couldn't find any record of a reason for the delay. The closest event was a fire/smoke damage on the bridge set, but thst was during the filming of "Bride of Chaotica!" (mid-Season 5 ep). Not sure if anyone knows the answer.