Although we don't believe in timing the market or panicking over daily movements, we do like to keep an eye on market changes -- just in case they're material to our investing thesis.

After a terrible January, investors had hoped to put a bad month behind them and start carving out new gains. But with the latest economic data showing a slowdown in U.S. manufacturing activity, the Dow Jones Industrials (DJINDICES:^DJI) didn't cooperate with that plan, starting out February with a 150-point drop as of 11 a.m. EST. Telecom stocks AT&T (NYSE:T) and Verizon (NYSE:VZ) led the way down in a broad decline among most Dow components, and Pfizer (NYSE:PFE) was the sole blue-chip component to post anything more than a minimal gain.

AT&T and Verizon both lost about 3% as AT&T offered new lower rates for high-data cell phone rate plans. Specifically, the new pricing applies to AT&T's shared 10-gigabyte monthly plans, and the company estimates savings at $40 to $100 monthly for family or small-business plans that cover four smartphones. The move could encourage some customers to upgrade, paying less than they would for four-gigabyte or six-gigabyte plans. But it also represents a departure for the wireless industry, which has historically relied on its highest-use customers to provide high-margin revenue. Investors are anxiously waiting to see whether Verizon will respond in kind, which seems likely given the direct assault that AT&T makes on Verizon's pricing scheme in its press release.

Pfizer bucked the downtrend, soaring 3.7% after announcing strong results in its PALOMA-1 study of breast-cancer drug palbociclib. The company said the drug met its primary endpoint of improving progression-free survival in advanced breast cancer among post-menopausal women. Even though the study is just a phase 2 trial, palbociclib received breakthrough-therapy status, giving it the potential for an accelerated approval process. Pfizer is starting to enroll patients for a phase 3 trial of the drug.

Given that many investors make monthly contributions toward their investment accounts, the first day of the month is often seen as having a positive bias. If the Dow doesn't recover today, it could be another warning sign that short-term traders in particular will pay attention to in charting their trading course for February.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.