Geiger Day is an event released during Cinco De Mayo Week 2017. It comes once a month like Surgery Day and All Howl's Eve and during this day, Geiger Counters take 15 minutes to charge and the Irradiated mod also lasts 15 minutes. A new supervillain H.R. Geiger spawns and new prizes are available, only during Geiger Day.
The supervillain H.R. Geiger spawns sometimes when you use a geiger counter. Normally, it will say, (someone) found a (name of item). Instead it will say, (Someone) was attacked by[H.R. Geiger. The villain then appears and you have to option to fight it in Card battles.