Timewalking in World of Warcraft is a recurring event that lets you revisit old expansions while leveling up and at max level. In Dragonflight, you can revisit six different expansions, and here's the Timewalking schedule.

In Dragonflight you can replay the following expansions:

You can’t choose which Timewalking you want to do, and instead you can only do the expansion that is active. It usually happens once or twice a month and lasts for a week. During that time you can collect Timewarped Badges and exchange them for items at Timewalking vendors. These items are usually mounts and cosmetics.

Additionally, you can complete various dungeons and raids on Timewalking difficulty and earn unique loot. You can carry that loot outside of Timewalking, but in most cases you can't upgrade it.