The fourth 節気 “sekki” division of the traditional Japanese calendar is:
vernal equinox, spring equinox
The second “kō” microseason within this division is:
(さくら はじめて さく)
“sakura hajimete saku”
First cherry blossoms
That makes 櫻始開 the eleventh microseason of the year!
It typically lasts 26~30 March.
This is #11 in a regular subseries of 365 Days of Japanese, where every five or six days I post about a Japanese microseason that has begun around the same date as the entry. I am taking all information from the list of 72 in this article. There is also a 36 microseason list here, which may be more authentic (rather than being based primarily on Chinese sources).
Image credit: The Star Online.