Epic has detailed the release date and time of the launch of Season 6 set for tomorrow.
Season 5 will conclude at 1am PST on September 27 and downtime for the 6.00 patch will begin immediately after. As soon as downtime begins, Epic will release patch notes detailing new additions and any other changes coming to Fortnite.
In previous seasons, downtime between updates typically lasted an hour, but updates between seasons are typically longer.
Any replays saved before the update will rendered useless after downtime concludes. Outfits with unlockable styles from the Season 5 battle pass can still be unlocked in Sesaon 6 as long as the skins were unlocked prior to the new season.
Source: IGN.com Fortnite Season 6 Downtime and Patch Information Detailed