What is the story behind Thor wearing a wedding dress to get Mjölnir back?
I often hear of a story of Loki taking Mjölnir away from Thor and selling it to giants. Thor then gets angry and makes Loki join him in wearing wedding dresses to enter the giants place unnoticed to get it back.
What is the actual story behind this and who were these giants and were they actually able to wield Mjölnir?
It was Thrymr, king of the jötnar, who stole Mjölnir. He then demanded the gods allow him to marry Freyja, in order to return it. Thor travelled to Jötunheimr to claim back his hammer, and he managed to sneak in dressed as a bride.
Loki wasn't involved in stealing the hammer, but he did accompany Thor, disguised as his bridesmaid.