Every single episode—every single episode–in every episode of Hogan’s Heroes, it was Winter. With snow on the ground.
What gives? 
They only carried out those hijinks during winter. Or they had a lot of clips with snow all ready to use repeatedly during the episodes, along with clips of POWs and camp guards in their winter uniforms.
I imagine it was because it meant that they never needed to redress the sets, nor worry about different costuming for the cast (other than their disguises when they snuck out of camp).
Edit: and, as Tripolar notes, it made it easier to re-use footage.
Not always a lot of snow. Sometimes just patches. Sometimes a blizzard.
But I do wonder about the tree stump tunnel entrance. Either the guards will see all the footprints, or someone will notice there’s exactly one tree stump with no snow on it.
I seee–nuthing!
They may have been trying to recreate the atmosphere of Stalag 17, at least at first. But I always thought it was odd that they had five years of perpetually cold weather. There was even snow on the ground in the D-Day episode. (The only one given a specific date.)
One glaring error in that movie, BTW, was that the Sun was always up during roll call at 06:00. It’s really, really dark at that hour if you’re in Europe in December.
To preserve a sense of continuity in reruns. You can watch the episodes in any order with no bouncing around the seasons. (Earth/tilt type seasons.)
So says a guy I watched on Youtube.
Early in production planning, it was decided to make it always be winter, with snow on the ground, and frost on the windows. This was to prevent problems with continuity, and to allow the episodes to be shown in any order (it also makes prison life seem bleaker, and being winter adds even more of a menace whenever Klink or Schultz are being threatened with being sent to the Russian front). Since much of the filming was done in the summer, the cast members had to wear coats and act cold, even when the temperature was over ninety degrees Fahrenheit (thirty-two degrees Celsius).
So somebody on Youtube was right, or can read.
They were actually being held prisoner in South Park.
Or that Nazi UFO base in Antarctica, as Congressional hearings are soon likely to reveal.
There are enough clues that the actual dates of a lot of the episodes can be deduced. Obvious dates such as D-day, Hitler’s birthday, the second Schweinfurt raid, etc, and inferences by what is happening on the Russian Front and things like that. They weren’t broadcast in real time order, but they can be reordered if someone wanted that.
Here’s some trivia not given above:
Carter was supposed to be single, but he always wore a wedding ring. This was because Larry Hovis was married in real life and refused to take the ring off.
Kinch gets a letter addressed to “James Kinchloe” in one episode. In another, an old friend calls him “Ivan” (the actor’s real first name).
It’s highly unlikely that Kinch would be in a Luftstalag at all, because the USAAF had no black aircrew. (Fighter pilots, yes; bomber aircrew, no.)
Bob Crane and Sigrid Valdis (“Fraulein Helga”) were married in real life. Sigrid was actually a brunette, not a blonde. (She wore a wig on the show.)
Alice Ghostley (the second “Frau Linkmeyer”) and Felice Orlandi (“Lt DuBois”) were married in real life.
Newkirk originally spoke with a Liverpudlian accent. Richard Dawson was ordered to switch to a Cockney accent because his Liverpudlian was too hard to understand.
It’s not obvious to me the arc of the episodes was meant to follow the arc of the war at all. They were in for the duration and the duration was indefinite. The war ebbed and flowed around them, but Stalag 13 was eternal.
Said another way, the fact various episodes referred to various real WW-II events does not mean the episodes were written, filmed, broadcast, or planned in that order. One could choose to sort them by real WW-II references, but I don’t know that that order is any more “real” or genuine than sorting by any other episode attribute or even just randomizing the order.
Series that are designed to have an ongoing story arc is a much later invention. We see that a lot now, as a way of encouraging audience engagement. Not back then.
I may well be wrong, and I’d be happy to be corrected if anyone has contrary knowledge.
Oh, don’t misunderstand. Just because they can be ordered, more or less, does not mean I think it was planned. It’s more like accidental continuity.
For example, in season 1 if Carter does a Hitler impersonation, and in season 3 Carter does a Hitler impersonation for the first time, obviously the second episode comes before the first, time line-wise. But that’s not a plan, that’s writers not watching their own show. 
Why was General Burkhalter in charge of Klink? Burkhalter appears to have been an army general whereas Klink was in the Luftwaffe. One call to Reichsmarchal Goering would have had Burkhalter running off with his tail between his legs. Goering did NOT tolerate army interference in Luftwaffe affairs.
Got it. Thanks.
Or maybe more like “a deliberate plan to just not have continuity problems”. Same with the perpetual winter. If each episode has no connection to the others beyond the recurring characters and the show’s overall theme / conceit, then continuity simply vanishes as a consideration. Cheap and easy.
Until / unless the show runs long enough that a real-world actor has a real-world problem like dying, getting pregnant, quitting in a huff, etc. Now the writers have a continuity problem to work around.
And somehow, Sgt Baker replaced Sgt Kinchloe, and no one noticed a thing! Not even a passing comment.
There’s actually much more “accidental discontinuity”. One day, Newkirk complains they “only have one lousy .45”, the next week they have a full arsenal in the tunnels. One episode, they use morse code to talk to London, next week they use voice (uncoded!). One week, Hogan’s code name is “Goldilocks”, the next week he’s “Papa Bear”, and always has been.
I love the show, but you can’t help to notice things watching them for the 20th time
WWII was just a setting. All of the Westerns were the same way. The westerns would all jump around to different times and include historical figures from different times, it was all just the Old West. They were all just individual stories, here’s another story, yes it’s from a different time but so what.
'cause everything else in the show is perfectly believable.
mmm