Why was Yom Ha‘atsma’ut a day early the past two years?

I can understand not wanting to have it fall on Shabbat day, since then Yom HaZikkaron would bleed into Shabbat, and even though it’s a not a fast day, it’s still a sad day which shouldn’t be on Shabbat.

But, what’s the problem with Yom Ha‘atsma’ut being ‘EREV Shabbat, with Yom HaZikkaron being on Thursday? There’s no problem with a happy day going into Shabbat, so why move it?

FoR: Zionist enough to treat these days in a religious manner.