The Fortnite team has announced that, due to stability concerns, the Survive The Storm update is delayed.
Previously, Fortnite's new survival mode was announced in celebration of the game's Early Access reaching one million players. The news was spotted on the Fortnite official Twitter account and, on the linked thread that will contain regular updates on the mode, this statement was made today:
We will update this story as soon as we know the new launch date for the Survive The Storm update. Feel free to master your crafting in the mean time with our Fortnite guides.