Besides the Autumn poets sing
A few prosaic days
A little this side of the snow
And that side of the Haze -
A few incisive mornings -
A few Ascetic eves -
Gone - Mr Bryant's "Golden Rod" -
The days are “prosaic” (normal or unassuming) in this case, because they’re in between seasons. 
 They don’t have the defining characteristics of summmer, or those of winter, and are more indistinct and mundane. Instead of attacking this normality, Dickinson argues that it should be celebrated. 
 The way she describes the fall as “prosaic” is important here, because it literally means “akin to prose, not poetic”, even though the autumn is the subject of the poem. 

Besides the Autumn poets sing 131