Vintage vs. Non-Vintage
Bottles in the average celebratory price-range are usually non-vintage, meaning they're made with blended Champagnes to achieve a specific taste. This way, Moët can make its Champagne taste like Moët should, every time. If there's no year printed on the label, it's likely non-vintage.
Vintage Champagnes exhibit specific characteristics associated with respective years and harvests. They can be enjoyed whenever, but they often cost more so you might want to buy for drinking in the early hours instead of dousing the room when the ball drops.