Reb eliezer I dont know the answer to your question on the Beis yosef. It is clearly a mistake I didnt see the avudraham inside Iwonder wha tit says there.
“ubi, you are contradicting yourself in two posts for Dec 5 . Meno is correct i.e.. Thursday night and maariv the previous day???”
no contradiction. (though a typo is possible someplace, please point it out) I grant it is confusing. i’ll break it down slowly:
There are two ways to say when we start vesein tal umatar.
This is true for any Jewish day.
Say someone asks when is chanuka this year? There are two possible answers you could correctly answer Monday Dec 23 ( ie the first day of Chanukah is Monday Dec 23) Or you could correctly answer Chanukah starts Sunday Dec 22. This isnt a contradiction, both are true. As you know Jewish days begin the night before so they are referring two 2 different things.
The start day of vesein tal umatar is no different.
We start 60 days after the Tekufas Tishrei (according to Shmuel, not according to scientists), with the day of the tekufa being day #1.
Tekufas Tishrei this year was Monday October 7 at 9 PM. Halachicly this is Tuesday (October 8). (much like halachikly Friday night is Shabbos)
you now count 60 days with day #1 being Tuesday Oct 8. If you do this correctly you will find Day #60 to be Friday Dec 6.
Thus Friday (Dec 6th) is the first DAY we begin saying vesein tal umatar.
Friday begins thursday night at maariv (when your calendar says Dec 5th) So we start on Dec 5th
2021* the chesbon is as follows:
Tekufas Tishrei in 2years will be Thursday October 7 at 9 AM.
you now count 60 days with day #1 being thu Oct 7. If you do this correctly you will find Day #60 to be Sun Dec 5.
Sunday (Dec 5th) is the first DAY we begin saying vesein tal umatar.
Sunday begins Motzei shabbos (Saturday night ) at maariv (when your calendar says Dec 4th) So we start on Dec 4 th
Meno is correct “I believe this year we start on Thursday night, December 5.”
This is because Friday (Dec 6th) is the 60th day after the tekufa so Friday is the first DAY of vesein tal umatar. Friday starts Thursday night Dec 5th so we start Thursday night Dec 5th
So in short your rule “can’t we say everytime Dec 4 except before a leap year Dec 5” is true
*I skipped 2020 becuase there is a twist, but the premise is true:
Next year the chesbon is as follows:
Tekufas Tishrei next year will be Wed October 7 at 3 AM.
you now count 60 days with day #1 being Wed Oct 7. If you do this correctly you will find Day #60 to be Shabbos Dec 5.
Shabbos (Dec 5th) is the first DAY we begin saying vesein tal umatar.
Shabbos begins Friday night at maariv (when your calendar says Dec 4th) So we start on Dec 4 th (of course we dont say vesein tal umatar at maariv Friday night but thats a side issue, and interesting question arises, if someone says the wrong shemoneh esrei and realizes during baruch alinu, the halcha is he finishes the beracha, should he say vesein tal umatar Friday night ? On the one hand thats the start point, on the other hand klal yisroel didnt start yet? IIRC IShei yisreol says he says vesein beracha because klal yisreol didnt start yet)