Describes a very enjoyable outing, event, set of circumstances, etc., which involves strong breezes and/or a lot of farting (think: the famously-uproarious mid-20th-century wind-breaking contest between whizzpopping-champions Lord Windesmere and Paul Boomer).
A social event which involves a lot of farting will usually be a lot more "winderful" if there are indeed strong fresh-air-delivering breezes present, also, since few people enjoy actually smelling a fart, even if they do get uproarious guffaws at letting them off and/or hearing other make them.
A constant state of being in Winder, Georgia. Occurs regardless of time of day, random meteorological events, train crashes, rioting, or mass chaos.
Jim: How are you today John?
John: I’m just Winderful today.
Jim: Even though there’s a tornado, the train hit an eighteen-wheeler, the rioters are destroying downtown, and anarchy has broken out?
John: It’s just another Winderful day.