What is the "demi-saison?"

 Literally, the half-season, or the season midway between winter and summer:  spring and fall.  In spring and fall, it's moderate weather, not too cold or too hot.

Many Bleuette patterns are intended for the demi-saison.  The first one, for example, was in 1910, issues 13 and 14, a fall dress.  

Next, a dress for spring, in the 1913 issue 11.  You can figure out from the date of the issue whether the fashion is intended for fall or spring.

This is 1924, issue 16 Vetement de demi-saison.

Her hat is from issue 17,

Chapeau printanier, Spring Hat.

Happy Spring!