The start of the second quarter usually begins with a flat to slightly down market for the first two weeks.

It's a well-known phenomenon: The usual explanation is that investors withdraw money to pay taxes. The good news is that the market tends to rebound immediately in the two weeks after April 15 (Tax Day is April 18 this year), with several sectors showing notable gains.

Our partner, data analytics firm Kensho, notes that since 2000 the S&P 500 has been down an average of 0.2 percent in the first two weeks of April, up only 41 percent of the time.

Two weeks after Tax Day, the S&P is up 1.7 percent on average, rising 75 percent of the time.

That's a swing of almost 2 percent in the S&P, on average — significant for a two-week period.

Several sectors seem to have notable moves: technology, financials, industrials and energy.

S&P Technology Sector

(April, since 2000)



First two weeks: down 1.1 percent

Two weeks after: up 2.2 percent



Source: Kensho

That's an average swing of more than 3 percent.

Financials also get a modest lift:

S&P Financials Sector

(April, since 2000)



First two weeks: flat

Two weeks after: up 1.7 percent



Source: Kensho

Industrials are flat, but come back strong in the second half, up 82 percent of the time:

S&P Industrials Sector

(April, since 2000)



First two weeks: flat

Two weeks after: up 2.2 percent



Source: Kensho

Oddly, this trade doesn't work with larger defensive sectors like health care and consumer staples.

What's going on? Besides the tax factor, there may also be some effect from earnings season, which begins right around April 15 (JPMorgan reports April 13 this year). Banks and industrials are among the first to report in those two weeks in the back half of April, so it's possible reactions to earnings play a role in the moves.

I wouldn't overanalyze this, just be aware of the effect.

Disclosure: NBCUniversal, parent of CNBC, is a minority investor in Kensho.