The fifth 節気 “sekki” division of the traditional Japanese calendar is:
pure and clear, clear and bright (solar term)
The first “kō” microseason within this division is:
That makes 玄鳥至 the thirteenth microseason of the year.
It typically lasts 5~9 April, so it began yesterday!
Image credit: Will the Swallows Return to San Juan Capistrano?