While we don’t have a specific date for the Black Friday sale set in stone yet, it typically runs from the Friday after US Thanksgiving until the following Monday (Black Friday through Cyber Monday). Black Friday this year is November 29.
We don’t have any plans for a Memorial Day sale, and I’m not aware of any other upcoming sales at this time.
You might consider looking into the PurpleAir Collective, which is a sensor donation initiative.
Hope that helps!