As the article says, Ruth was joking about it being "church for atheists." And it's on a Sunday because that's a day when most theists have a place to go for community (though there are a wide variety of types of events at the Hub, which are held throughout the week; the Sunday event just tends to be one of the larger events). 

As for the nature of God, atheists are aware that there are many different concepts of gods--but there is not evidence to support the existence of any of them.