The first 節気 “sekki” division of the traditional Japanese calendar is:
the beginning of spring
The first of the three “kō” microseasons within this division is:
“harukaze kōri o toku”
East wind melts the ice
That makes 東風解凍 the first microseason of the year!
It typically lasts 4~8 February.
Image credit: Fairegarden blog.