My hunch is that it was sometime in the early-mid 1990s, during the shift in the entire zeitgeist.
Halloween changed from a carnival holiday for children, testing and proving the strength of guest-host relations within the community, to a one-upsmanship contest among grown-ups to see whose costume is the most ironic, clever, and effortful, or which female can whore for the most attention while giving away the least interaction to get it.
The Fourth of July used to be a display of patriotism, at least of neighborhood unity (e.g., block parties), but now is Nuclear Family BBQ Day. Trust has collapsed so low that only the closest blood relatives are allowed into the circle.
Christmas used to focus on the supernatural and gift-giving. Now the focus has shifted from Christmas to Black Friday, a purely materialistic orgy of self-indulgence -- buying stuff for yourself at deals that only come once a year.
New Year's I don't remember being a big holiday in the '80s. I was too young to go out nightclubbing or anything, but I still would have noticed my parents and others getting psychotic about it, if that had been common. It seemed like a lower-key version of Halloween, for grown-ups, where the point was just to get out and join the revelry of the crowd. Also, to make a New Year's Resolution, which was for the benefit of others -- being kinder, losing weight (which most do out of shame, not for personal health benefits), spending more time with friends and family, etc.
Now New Year's has become It's All About Me Day. My brothers and I were trying to figure out what we're going to do for it, and it's nuts how status-conscious all of the options are, how expensive, and inevitably how boring. Pay $175 for an evening of tapas, wine, and competing against the other customers to see who can use the most authentic Castilian accent when ordering. Or $90 for some lame DJ in a yuppie / hipster part of town. No matter what it is and how much it costs, it's now very important that people know What You're Doing for New Year's. It broadcasts a signal about who you are, one that everyone else just has to know, just like your clever Halloween costume.
Things have been heading in this direction for awhile, although the heights they've reached by now are still perplexing. What ever happened to rounding up some friends in a car, getting drunk in public -- a park, a city street, or wherever -- and singing and carousing with whoever else happened to also be there? Not like it's vanished, but it's moved toward a self-congratulating holiday. And not only for SWPLs as I may have suggested above, but including folks who will enjoy bragging about "poppin' bottles," etc.
People don't seem to take their New Year's resolutions seriously anymore either. Maybe they don't make them at all, or give little thought, or from the outset don't intend to give them much effort during the new year. It would ruin the feel of "look at how unique and exclusive my plans are" surrounding this Self-Esteem / Statement of My Identity Day.