I would confirm that the missionary circle takes place at the Finches' home under the care of Aunt Alexandra during the end of summer before the kids go back to school, in August.
Aunt Alexandra was hosting this event so that the ladies of Maycomb who helped support the missionary J. Grimes Everett would have the opportunity to learn about where their donations are going and what the donations are doing. However, we find that through Scout's eyes, the missionary circle is about much more than that. They gossip and participate in terrible hypocrisy.
The significance of this event in this time and this place is that it allows Scout to discover some disturbing information in the kitchen about Tom Robinson. Had the meeting been at someone else's house, Scout might not have learned the information because she likely wouldn't have gone. Being the end of summer, the trial is still fresh in everyone's minds and it is almost too much to bear for the Finch family. This build great suspense.
The womans missionary circle meeting was held in the home of Atticus Finch, I think to the consternation of Scout.
As Chapter 24 starts...( the second paragraph) August was on the brink of September..sounds like the end of summer to me..:-)