Church clothes are your best bet. The Orthodox Church is a holy place. Please use your best judgment. The groom will likely be in a nice pair of jeans, so don't go crazy! But please don't show up in shorts, a tube top, and flip flops....especially if you are a dude. :-)
There is a parking garage next to Relevant Church!
Go watch My Big Fat Greek Wedding....seriously. You can also find a cheat sheet here: https://tinyurl.com/michaelandtanya