So far in April stocks have had a great month, with the S&P 500 and Dow Jones industrial average notching all-time highs. But things may be about to take a rockier turn for investors from here on out.

That’s in part because, although April generally tends to be one of the best months for the stock market, historically most of those gains are racked up in the first half of the month, per LPL Financial data.

Indeed, LPL’s Ryan Detrick tells Fortune that “this year, we’ve seen the pattern play out the first three months of really strong equities [in] the first half of the month and then some weakness the second half of the month,” and that “the last 10 days or so of April are kind of a little more rocky,” he says. What’s more, he notes that “the worst six months of the year historically is May through Halloween, and that is obviously on the horizon after just a record-breaking 12-month rally.”

Adds Detrick: “The calendar could potentially be one of investors’ biggest near-term worries.”

That would certainly be a turnaround from what has been a smooth stretch for investors, with volatility (as measured by the VIX index, or fear gauge) falling to pre-pandemic levels. That has prompted some firms like UBS to caution that, “with markets continuing to hit record highs, some investors are concerned that the recent decline in volatility may not last,” predicting “periodic bouts of higher volatility in the near term,” UBS Global Wealth Management chief investment officer Mark Haefele wrote in an April 9 report.

But more than that, those like Detrick note there’s so much bullishness all around that that might be its own warning sign: “The No. 1 concern, at least for me, is just everything is really good.” With strong economic data coming out and an expected “blowout” Q1 earnings season kicking off, “the contrarian in me wonders how much of this good news is priced in,” Detrick says. That’s why he wouldn’t be surprised if stocks saw a correction (technically a 10% selloff) or two in the coming months.

Meanwhile with investors pushing stocks up to all-time highs, valuations keep soaring (the S&P 500’s trailing P/E is a heady 33 times earnings). That in itself is making some analysts and market watchers alike wary.

So far on Monday, stocks are down across the board, with the S&P 500 down roughly 0.6%, the Dow off around 0.4%, and the Nasdaq over 1.1% lower in afternoon trading.

But neither the historically weaker seasonality nor the prospect of higher volatility in the near term are putting analysts off their bullish longer-term expectations. “It’ll be scary when it happens,” argues Detrick, “but investors need to start preparing for it…and use it as an opportunity to get things a little cheaper, which again is the name of the game in [what] we still think is an upward bull market.”