2017 was another great year for the Dow Jones Industrials (DJINDICES:^DJI). The average posted a gain of roughly 5,000 points -- its biggest point advance ever -- during a period that many expected would bring an end to the long bull market. The year also featured an almost unprecedented lack of volatility: Both its daily advances and declines were typically modest.

Even though the stock market featured long runs of minimal movements, there were some big days for the Dow. Its biggest daily gain -- 332 points -- came on Nov. 30, and set the stage for an end-of-year rally that put a cherry on top of a rewarding 2017 for investors. But what powered the Dow's best day of the year?

^DJI data by YCharts

332-point gains aren't what they used to be

Perhaps the most important thing to realize about the Dow's strongest performance during 2017 was that, in terms of a percentage move, it wasn't that big of a deal. Back when the Dow was lower, any triple-digit move was a big deal. With the average now flirting with the 25,000 mark, a 100-point move works out to about 0.4%, which isn't a big change at all. Even the 332-point gain on Nov. 30 amounted to just 1.4% -- which pales in comparison to the ups and downs that the average has seen in previous years.

Also noteworthy was the fact that the jump didn't come in response to an earlier major decline. In past years, the best days often came right after bad ones, with investors initially responding to bad news by selling off their positions, only to re-buy in a relief rally once their worst fears end up not coming to pass. This time, though, the Dow's big move followed two triple-digit rises, and capped a month in which the Dow gained nearly 700 points and closed above the 24,000 mark for the first time.

Numbers 2017 in white light with fireworks exploding around it.

Image source: Getty Images.

The big event: tax reform

The catalyst for the Dow's best day of the year came from Washington. After months of failing to overcome internal party discord, Republicans in Congress desperately needed a legislative victory. Earlier efforts to repeal the Affordable Care Act had come to naught, and so GOP lawmakers turned their attention to a tax overhaul.

In mid-November, the House passed a tax reform bill that featured sweeping changes, including dramatic reductions in the corporate tax rate. Yet investors weren't nearly as confident that the Senate would follow suit, because of the much narrower margin of Republican control in the upper chamber, and the vocal disagreement to aspects of the proposal from certain swing lawmakers within the party.

As the month came to a close, however, the Senate started to reach agreement about what a tax overhaul package might look like. As the pieces fell into place, investors responded by bidding up stock prices. By Dec. 1, the Senate had taken up the bill for a vote, and late in the evening session, the measure passed by a narrow 51-48 vote.

The big lesson from the Dow's best day

Even after the big Nov. 30 gain, the Dow wasn't done for the year. After all, the Senate's bill required reconciliation with the House version, and many remained doubtful that the differences between the two proposals could be bridged. Yet in the end, that's exactly what happened, and along the way, some additional optimistic days for the Dow brought the average a lot closer to 25,000 than most would have imagined possible at the beginning of the year.

The key lesson for investors from Nov. 30's gains for the Dow is that if you wait for good news to be certain, you can miss out on substantial profits. Markets adjust to good news incrementally. By following a long-term investing strategy, you'll be invested in the market no matter when favorable things happen, and reap the rewards accordingly.