“Don’t fight the Fed” is a mantra that investors know not to ignore—and their adherence to it will be tested next week.

After raising interest rates in 11 of the past 12 meetings, the Fed is widely expected to keep rates on hold this month. That means interest rates are forecast to remain between 5.25%-5.50% until at least November.

But deciding what to do with interest rates is only one part of the Fed’s puzzle. The central bank must also give the market proper guidance on what to expect moving forward.

Expectations drive Wall Street—and, by extension, the economy and your personal finances.

September has always been a critical month for setting expectations. The stakes are higher this year because the Fed is doing something it hasn’t done in four decades.

September carries more weight

The Fed’s upcoming meeting is important for two reasons.

First, the Fed will release its projections for GDP, inflation, and interest rates. Investors use these projections to determine where interest rates could be in 12 months.

Right now, Fed officials are conflicted about inflation—making it harder to trust their guidance on rates.

Some officials, like Cleveland Fed President Loretta Mester, believe rates should increase to provide an insurance policy against inflation.

“We’ve been underestimating inflation,” she said in an interview last month. “Allowing inflation to be up for longer does carry a cost for the economy.”

Others, like Boston Fed President Susan Collins, believe it’s pointless to only focus on interest rates and not the overall economy. “This phase of our policy cycle requires patience,” she said, referring to the need to hold off on additional rate hikes for now.

Talks of “patience” lead to the second reason September is so critical for the Fed: seasonality.

Historically, September has been the worst month for stocks. Since 1928, the S&P 500 Index has declined 52 times in September—more than any other month, according to Yardeni Research.

There’s no iron-clad explanation for why stocks perform so poorly in September.

The so-called “September Effect” probably has something to do with investors needing to lock in gains or tax losses after the summer holidays—or the need to sell assets to pay for children’s education expenses.

Fed officials, therefore, are usually extra cautious about how they lay out expectations. They know investors hang on to their every word.

Bracing for near-term impact

What the Fed does with interest rates impacts an array of borrowing costs, including credit card and mortgage rates.

Americans have racked up $1 trillion in credit card debt—a figure expected to increase during the busy holiday season.

With mortgage rates at 22-year highs, consumers can’t tap into their home equity without sucking up huge refinancing costs. That means many Americans will rely on credit cards and personal lines of credit this Christmas.

If Wall Street deems the Fed’s policies and projections to be negative, history tells us a sell-off in stocks is likely—impacting everything from 401(k)s to mutual funds.

Although one policy decision won’t make or break personal investments, the Fed’s September meeting could result in a lump of coal for Christmas.