It is an international event where Ham radio operators all over the world set up temporary "emergency" communications systems and try to contact as many other Ham stations doing the same thing over a 24 hour period. This is a disaster preparedness exercise but also a lot of fun and it probably taking place somewhere near you so you can stop by and see what is going on.
Even though amateur radio is over 100 years old, Ham radio has actually grown over the last few yearsthrough the DIY movement and the integration of radio and the Internet. Efforts in these spaces will likely be on display at the field day sites.
I’ve been a ham since 1972 or so and my current call is AD5EN, so feel free to ask me if you have questions.