From when until when are you allowed to make the bracha,and what is optimum?
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The bracha can be made from the time the tree starts to bloom, regardless of the month (Mishna Berura quoting "Acharonim"). Lechatechita it should be said before the fruit grows.
The Mishna Berurah sides that, bediavad, if you did not see the blossoming and the fruit started to grow, you can rely on those who say you can make a bracha until the fruit is fully complete (minimally edible?). It sounds like if you saw the blossom and passed up the opportunity, you should not rely on this opinion. The Aruch haShulchan does not side with that qualification.
A lot more sources posted at Dose of Halacha