Lateral Thinking Puzzles. Let's do it again!

Was it October 15 when he said this?
Is it relevant what language they were speaking?
Is the specific significance of that date known to the (appropriately-educated) world at large?
Is this related to the conversion from the Julian to the Gregorian calendar?

Thanks all!

Would the answer be obvious if you gave the full name?
Is the S. short for Stalin?

Would the puzzle work as well if it were some other arbitrary date at an arbitrary point in the year?

Did somebody previously ask “What is the 288th day of the year?”

tks

Is there a pun or any word play involved?

No pun or word play, all participants are adult human beings.

Was “October 15th” pertinent to the preceding conversation?
Was “October 15th” a guess?
Had something happened on that date to the person who didn’t like the joke?

Joe S. was at the bar with some old friends when he suddenly blurted out “October 15th!” Half of his friends laughed and raised a glass in appreciation, half were confused, one didn’t like the joke at all. What was the joke about?

As a nudge, the date is not the only part of the puzzle that matters.

Was the date relevant to something that happened a set or predictable time period before or after Oct 15? Such as being conceived 9 months before or giving birth 9 months after?

NO
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Did Oct. 15th also reference a certain location?
Did it reference a certain event?
Any astrological significance?

Was there some sort of social group division of the people who were confused versus the people who laughed? By Age? Sex? Race? Something else?

Did Joe say the specific words “October 15th” and only that?

Was it in reply to a question someone had asked?

If so: was it the right answer? Did it take Joe some time to figure it out? Did the people who laughed already know that it was the right answer?

It was A right answer not necessarily THE right answer. He knew the answer by heart. They immediately recognized it as an appropriate answer, but may have answered it differently themselves.

Did the preceding question actually ask when something took place?
Was the subject matter concerning a certain accomplishment? (like “when did you lose your virginity?”)

NO to both.

Was the division between the reactions based on politics?

NO, but this is a good avenue of questioning.

Were they discussing sports?
Music?
TV show?
Movie?
Celebrity?
Local topic?