I've always wanted to wield a lightsaber while dressed as a medieval knight, and now I can live out my dreams, but only for the duration of Star Wars Day.
The change causes weapons to turn into a lightsaber, and while the effect is somewhat lost on weapons like battleaxes, with broadswords it's pretty awesome - so much so, players are now rallying around on Reddit for the feature to be a permanent option.
It seems this was planned in advance, as the code for it appears to have appeared in the Year 3, Season 2 update. Sadly, it seems unlikely to stick around long after today, but perhaps it will appear as a feature of some kind based on the demand in the next update.
In the mean time, you should give it a go quickly, before you miss out - or check out the many images and videos floating around the internet, I guess.