Derech
I don’t think the same dates are universally observed.
This is a corrrect observation:- This year some places shall observe Yom ha’Atzmaut Bi’Zemano on Friday Iyyor 5th, whereas others shall be Makdim Yom ha’Atzmaut to Thursday Iyyor 4th.
Then a week later, some places shall be reciting Tachnun on Pesach Sheni, whereas other do not, and some only by Mincho [Zeman Shechitas Korban Pesach Sheni] omit Tachnun.
Then 3 weeks after that, some places re-commence Tachnun day
after Isrru Shovu’os:- Sivvon 9th, whereas other places don’t re-commence Tachnun, until Sivvon 13th.