See, that sounds slightly off to me, too. Should be “when” in my dialect.
That’s right. If it stops raining several times that day, back you go to the zoo.
Ok. Well I suppose we’ve discovered a dialect difference. At least we’ve shined our collective flashlights into all the nooks and crannies. And agree there’s a real difference.
Another question, probably unanswerable, is whether there’s any support for my ideas outside my own idiosyncratic usage.
Growing up I heard people use it when referring to events in the past.
“Whenever it stops raining” sounds totally normal to me. “When it stops raining” doesn’t sound wrong but to me that has a more predictable finite sense to it like you know it will happen. “It will stop raining and we will go to the zoo then,” whereas “whenever” says to me “if it does stop raining, we’ll go then. Whenever it happens, I don’t know.”
Whenever is more unpredictable.

See, that sounds slightly off to me, too. Should be “when” in my dialect.

“Whenever it stops raining” sounds totally normal to me. “When it stops raining” doesn’t sound wrong but to me that has a more predictable finite sense to it like you know it will happen. “It will stop raining and we will go to the zoo then,” whereas “whenever” says to me “if it does stop raining, we’ll go then. Whenever it happens, I don’t know.”
Yep, same here - “whenever” makes perfect sense to me in your usage. I grew up presumedly using the same Chicago-area dialect as pulykamell, but living in drizzly Seattle for decades, “whenever it stops raining” is a pretty common phrase!
Could be just me, then.
Girl I knew who used ‘whenever’ like that was from South Carolina.
The more I repeat the two versions to myself, the more they sound normal to me now. So it probably was just me.
One of our junior programmers uses “whenever” instead of “when” all the time. He’s a terrible writer and speaker in general and his reading comprehension is very poor. I hadn’t considered that dialect could explain his use of “whenever”. His writing would still suck even if he used the correct words.
Aside:
How could someone whose reading comprehension is poor possibly work in IT when the very definition of the job is read carefully written specifications, understand them perfectly, identify every gap, oversight, or inconsistency in them, and create software to perfectly duplicate the intent.
ISTM poor reading comprehension would be utterly disqualifying. It sure was in my dev shop back in the day.

One of our junior programmers uses “whenever” instead of “when” all the time. He’s a terrible writer and speaker in general and his reading comprehension is very poor. I hadn’t considered that dialect could explain his use of “whenever”.
I think the two words do connote different meanings, dialect or no dialect. Some people just get stuck on using certain words.
Adding to what people have already said, I feel there are two distinct usages for whenever.
One is to describe a situation where a cause triggered an event on multiple occasions - As a child I got sick whenever I travelled on a boat - as opposed to a situation where a cause triggered an event but not on a recurring basis - I got sick when I was on that Mediterranean cruise.
The other usage is to describe an even that will occur at some vague and undefined time period. In some cases, the time period might be so vague as to suggest the event will never occur. I’ll mow the lawn whenever I get around to it.

One of our junior programmers uses “whenever” instead of “when” all the time.
He’s probably also a terrible coder. Random SQL “when”:
CASE
WHEN condition1 THEN result1
WHEN condition2 THEN result2
WHEN conditionN THEN resultN
ELSE result
END;
Yesterday I watched the Netflix documentary Trainwreck: The Astroworld Tragedy. One young man from Houston said something along the lines of “I was 18 whenever Astroworld happened”.

“I was 18 whenever Astroworld happened”
Which would make sense if he had said something like, “I don’t remeber exactly what year that was, but I was 18, whenever it happened.”