RE
sure
There is always a 2 day difference between the day of the week when the tekufa is and the day of the week that we start saying vesen tal
for example if the Tekufa was Sunday then vesein tal is At Maariv for yom revvi (what we call Tuesday night which is the start of yom revii)
This year the Tekufa was Tuesday (halachickly) so vesein tal is begun 2 days later in the week, at the start of Friday ie Thursday night
in 2021 The tekufa will be Thursday so vesein tal is begun 2 days later in the week at the start of sunday ie Motzoei shabbos
shatzmatz
Your first paragraph is correct.
I have a questionon your second paragraph . do you have a source that the Julian calendar began March, I was under the impression it began in January . And even if they did the word “היה” still doesnt fit well.
My thought was that maybe he means if this year IS a goyish leap year, ie 5780 has an extra day in February, thoug hthis explanntion, even if more accurate, still suffers the same flaw. as היה doesnt quite mean “is” though perhaps this is better than “will be” which it clearly doesn’t mean