A glass of champagne has the same alcohol content as a glass of wine or your basic cocktail.  But it's the bubbles (gas) in the champagne that causes it to get absorbed faster in to your stomach and into the bloodstream so that most folks get tipsy or drunk much more quickly than usual.

Because the champagne making process involves two fermentations, it contains more of certain chemicals, called "congeners" that make hangovers worse.

Personally, I've always avoided champagne ever since that first time sip gave me a toxic headache that I don't need - no matter what the special occasion is.