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" -
And Mr Thomson's "sheaves."
Like the  line which follows,  this is a adjective which gives the reader pause for thought inserted craftily into a throwaway phrase– “A few mornings”. 
 
 Cold mornings = frosted dew 
 We often here of the cold being “cutting”, but Dickinson heightens the register and deploys the Latinate “incisive”, which derives from “cutting” but has come to mean mental cutting– it’s used to describe particularly intelligent observations. The word deepens this line’s symmetry with the following one.
Besides the Autumn poets sing 131