Well, that didn't take long.

After the S&P 500 lost nearly 13% over the last 14 sessions of September, stocks have come roaring back to open the fourth quarter, posting their biggest two-day gain since the depths of the pandemic on Monday and Tuesday. The broad market index jumped 5.7% higher over the first two sessions in October, even though there was no major catalyst for the movement.

If the stock market feels like it's going on and off like a light switch, there's a good reason for that. The Federal Reserve is dominating market sentiment, and predictions about the Fed's upcoming fed funds rate decisions can turn on just a hint that the economy is weakening. 

For example, on Monday, the Institute for Supply for Management's Manufacturing Purchasing Managers Index came in at 50.9, indicating a slight expansion, but at its lowest level since May 2020. Worse, leading indicators like new orders were actually down from the prior month, a sign that the economy could be slipping into a recession. Also on Monday, a U.N. agency warned the Fed and other central banks that continuing interest rate hikes could push the world into a prolonged recession.

On Tuesday, stocks rallied again after job openings fell by 10% from July to August to 10.1 million, its lowest level in over a year. That data point is likely to put an even brighter spotlight on Friday's jobs report.

In normal times, the monthly jobs report is closely watched as an indicator of the health of the economy. However, with market sentiment hinging on Wall Street's latest guess of the Fed's next move, this week's jobs report takes on even more importance.

Up is down

In a stable economy, investors like to see steady growth from the job market and low unemployment. However, the Fed's aggressive interest rate hikes to combat inflation have disrupted the normal market mentality. In commentary after the central bank's latest hike on Sept. 21, Chairman Jerome Powell said that the most important goal of Fed policy was to bring down inflation, even if that meant driving unemployment higher or causing a recession.

That means the sooner that the economy buckles under the weight of higher interest rates, which have increased by 3 percentage points since the beginning of the year, the sooner the Fed is likely to pump the brakes on its rate hikes.

That's good news for the stock market for several reasons. First, rising interest rates effectively makes money more expensive. It makes it more costly for businesses to borrow money, slowing down growth, and it raises payments on variable-rate debt. In some industries, like homebuilding, climbing interest rates have already had an impact on consumer behavior.

Second, rising rates tend to encourage investors to pull money out of the stock market and put it into the bond market to benefit from higher yields.

Finally, higher interest rates lift the discount rate in financial analysis like the discounted cash flow model, making future earnings worth less. That explains why growth stocks, especially unprofitable ones, have fallen sharply this year.

What to look for in the jobs report

According to Factset, economists are expecting an increase of 250,000 jobs in September. While that still represents job market growth relative to population expansion, it would be the slowest month of job growth since December 2020 as the labor market has steadily clawed back the millions of jobs lost during the pandemic. Total employment only topped pre-pandemic levels last month, according to the St. Louis Fed.

In addition to the number of jobs added, investors will be also be focused on the unemployment rate, which is the number of people looking for work divided by the size of the labor force. The unemployment rate can rise either because people lose their jobs or because more people decide to look for work. In August, the unemployment rate rose from 3.5% to 3.7% as the labor force expanded, a sign that more Americans were looking for work.

The sharp drop in job openings in August is a good sign that the labor market weakened in September. Still, we won't know the official tally until Friday at 8:30 a.m. ET.

If job growth comes in below 250,000, don't be surprised if stocks surge once again.