@Lizzie, a couple more thoughts on this: I suspect what originally happened is that (in affected accounts) Booking Window settings started behaving as if "Dates unavailable by default" was the selected option. Note that my settings at the time were "6 months into the future" as previously noted. "6 months" is what was showing in my Booking Window options pulldown as well.
In addition to the reports of Calendar blocking behavior here, I also saw several tweets from hosts to @AirbnbHelp on twitter about the same issue. Also saw a tweet to @AirbnbHelp from several hours earlier Tuesday morning (perhaps from Monday night?) from a guest, reporting that they weren't seeing availability for *any* property in Tokyo... Just mentioning this in case it's helpful/related in tracking down the source of the issue.
As I and others noted in the thread, it is *possible* that there was some sort of interaction going on between imported synced calendars and the Booking Window settings, but that didn't seem predictable or reproducible. Personally, I suspect that changing Booking Window settings was causing more-or-less random/incorrect results... But it's worth pointing out that some users observed that disconnecting (deleting) then re-connecting their imported calendar(s) made the Booking Window settings behave *differently* (if not correctly) after the re-import.
Best Regards,
Keith