I don’t claim I know either. I’m not a Talmudician, not a Rabbi, never claimed to know.
I think it’s called Minhag avosainu byadeinu. And that may have been what kept us a a nation, unwavering, throughout all the generations. We followed our sages regardless if we (and them) knew the right day. ????? ?????? ?? ???? ???? ?? ????. So it follows naturally that we follow our mesorah too, even in Eretz Yisroel. We’re still listening to our Sages.