https://www.gamespot.com/articles/monster-hunter-world-last-chance-for-kulve-taroth-/1100-6458670/

Monster Hunter World's recent 3.0 update introduced a new Elder Dragon to the game, but it won't be available for much longer. The first Kulve Taroth Siege ends on PS4 and Xbox One tomorrow, May 3, giving players only a little more time to obtain its new weapons and armor.

The Siege runs until 4:59 PM PT / 7:59 PM ET / 11:59 PM UTC, after which time it will be taken out of the event rotation. If you haven't had a chance to participate in the quest yet, Capcom says it will return to the game "soon," although the publisher hasn't indicated just when it will be available again.