In what season of the year does the play The Miracle Worker take place?
I am not certain that the season is ever mentioned. I have seen several adaptations where the beginning scene is almost always set in the fall (indicating that, by the season which is not as happy-go-lucky as summer and is leading us all into the gloom of winter, the next years for the family will be turbulent at best).
In the version of the play I have, it clearly mentions the 1880's as the decade, and Tuscumbia, Alabama as the geographical location. The time of day is night, and the family, along with the doctor, are standing around Helen's crib and marveling at her survival. We then jump to several years later where the child has become a real horror to live with as denying her anything causes violent outbreaks and tantrums.
This, of course, is where Miss Annie Sullivan is introduced and the family learns to deal with Helen in new ways.
I think if I were planning this play, I would set it in mid to late November...sweater weather, but not with snow on the ground just yet. Perhaps as the audience realizes with the mother that Helen is suffering adverse reactions from her illness, then the snow could fall and the wind could fiercely blow.