Dress code, revisited

I am going to a reception tomorrow, and the invitation says "Lounge suit". My, isn't internet marvelous! I get drowned in the history of clothes, one article more fascinating than the other. And to think that I have lived my whole life without this essential knowledge. I own an evening gown that I have used twice, but otherwise I have always managed without going deeper into these very intricate details. Fortunately, there is a very easy guide. My only concern is: looking out of the window, I am not sure that the climate is temperate. But I won't test the option "cold" in case it means woollen underwear.