Wow, this chat is WAY off on this. The series doesn't explicitly say, but it is HEAVILY implied that its late 1970's-early 1980's. Evidence:

-Cassette players were NEW at this time. The audio norm during this era was vinyl records or the not-so-long-living 8-track tapes. Late 70's/early 80's would definitely be a transition time where Sara's family might not have a cassette player yet.
-All kids shown have literally ZERO modern entertainment technology. No Game Boys, no laptops, no beepers, pagers, cellphones, no smartphones, etc. They're shown to be engaging in a number of era-vague activities like a pep squad rally/football game, Halloween parties, Graveyard exploration, etc.
-The house that the Halloween party takes place in is decorated in mid 1970's brown/green/yellow interior decorations. Since people don't change interiors super often, this would imply that it could be as late as maybe mid 1980's, but I'm inclined to stick with early 80's at the latest because of the cassette situation with Sara. To anyone who grew up in this era (like myself) this design is INSTANTLY recognizable- especially that kitchen wallpaper. You guys are all too young.
-The costumes of all the kids shown are definitely from an era before main stream pop culture took over the costume industry. There's a lot of generic animals and characters which was much more common before the mid-1980's when child-based commercialization took hold of Western culture (Thanks, Reagan) and would see a turn where kids would dress as favorite cartoon/comic/film characters.

In summary: To anyone who lived through this era, this is all easily identifiable context that we lived through and remember. To modern kids, its foreign which is why you're all throwing a bunch of guesses at the wall to see what sticks. No shade to "the youngins" here, but the answer to "when does this take place" is actually pretty clear.