The three main stock indexes in the U.S. — the Dow, the S&P 500  and the Nasdaq composite  — all hit record highs on Thursday. That’s a feat that hasn’t happened since Dec. 31, 1999, which gave a lot of investors a lot to celebrate at their New Year’s Eve celebrations.

The question now is: What’s causing this party, and how long will it last?

Part of it could be an impression of strong retail earnings and a late surge in oil prices this week. Bond yields also continue to stay very low. The Federal Reserve’s actions in keeping interest rates low for so long could be helping the push toward equities.

There’s disagreement over whether gains will continue even higher or if we’re at the tail end of them.

Last time this happened, it didn’t last that long. Over the course of the next year, the Nasdaq composite dropped off by more than half.