Sealcoating season typically starts when evening temperatures get to 50 degrees. This allows sealcoating applications to adhere properly to asphalt surfaces and gives the best result. That means that typically sometime in spring is when we can start sealcoating driveways and parking lots.
While fall oftentimes has temperatures that sealcoating may be performed in, doing so may result in leaves falling or blowing into the wet sealer causing problems with the overall finish.
What it comes down to is the earlier in the year you are able to have sealcoating done the better to prevent some of the issues that fall brings.
This also gives the sealcoating application plenty of time to cure properly before winter weather arrives.