The Bloody Harvest is a limited time event added to Borderlands 3 to celebrate Halloween, and runs from October 24 until December 5. So you have a decent amount of time to complete the short event, and the first thing you want to do is to figure out where and how to start it. In order to begin it, you will need to talk to an NPC called Maurice.
Maurice is a new NPC that can only be found after you have completed the "Taking Flight" mission and unlocked Sanctuary for the first time. This is not too far into the game, so most people should have already completed this mission by now. Anyway, once you have unlocked Sanctuary go to the top deck next to the Mayhem activation panels. He should be fairly easy to see, and just talk with him to begin the Bloody Harvent quests. When you talk with him, you will begin the event, and if you ever want to redo it, just talk with him again at a later point.