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

What: Shares of bank holding company Hudson City Bancorp (Nasdaq: HCBK) climbed 15% today after M&T Bank (NYSE: MTB) agreed to acquire it for about $3.7 billion.

So what: The cash-and-stock deal values Hudson City at roughly $7.22 per share and represents a 12% premium to its closing price on Friday. M&T Bank is making the move to expand its reach in the Eastern U.S. (particularly in New Jersey), and judging by its own stock's 5% pop today, Wall Street seems pleased with the price it's paying to do it.

Now what: M&T expects to gain about $25 billion in deposits and $28 billion in loans from the merger, with the deal being immediately accretive to the combined company's capital ratios and earnings. "As we combine Hudson City's attractive retail network with M&T's full service commercial banking suite, our stakeholders will participate in the growth of one of the nation's strongest and most successful banking franchises," Hudson City Chairman and CEO Ronald Hermance Jr. said. So while Hudson City shares might be all popped out, M&T's expanded Eastern presence might be a strong driver of market-topping gains going forward.