The Greek word that the Apostle Paul used in this verse is "kwmoj". This word describes parties or feasts with the intent of drinking all night long. While drunkenness is about an individual opening his or her body to danger, revelling is celebrating that decision.
Galatia was incorporated into the Roman Empire around 25 B.C. and it was roughly 75 years later that the Apostle Paul wrote his epistle to the Galatians. At the time that Paul's letter was written, it was a common practice for the Galatians to honor the god Bacchus through orgies and drinking through the night.
Revellings represent public blasphemy against God. They also desecrate the temple of the Holy Spirit - our bodies (Rom. 12:1).
"What? know ye not that your body is the temple of the Holy Ghost which is in you, which ye have of God, and ye are not your own?" - 1 Corinthians 6:19
The other aspect of kwmoj is that these parties are riotous. These are loud festivities that go beyond a simple house party. They are events that are organized and spread into the streets, travel to different buildings, and gather more people into their fold throughout the night.
Revelling can best be understood as this: drunken evangelizing that showcases the fun of immorality.