Have you checked the $Westpac Banking Corp (WBC.AU)$ share price today? Before you faint at the drastic decline, there's a rather simple and relieving explanation.

Shares in the banking major are 5.5% lower to $26.36 as we head into the afternoon. If it's any consolation, the $S&P/ASX 200 (.XJO.AU)$ is 0.74% worse for wear. Aussie investors are skittish today as numerous ASX retail shares reveal underwhelming sales updates, further demonstrating weakness in consumer spending.

Back to Westpac... what is the catalyst sending this $92 billion bank to the bottom of the barrel today?

Dividend deadline arrives for Westpac shares

Don't worry, it's nothing catastrophic. It's merely the ex-dividend date.

This is when the company's shares begin trading without the claim to the next dividend payment. All shareholders on Westpac's share register yesterday will snag the 90 cents per share fully franked dividend, but anyone added to the list today will not.

As usual, the share price is falling to reflect the absent dividend value.

In this case, Westpac's dividend has a grossed-up value of about $1.29 after franking credits. Yesterday's closing price was $27.89. Take the former from the latter, and you end up with $26.60 — roughly in line with the current Westpac share price.

Shareholders can expect this interim dividend to land in their account on 25 June 2024.

CBA canary in the banking coal mine

There's a chance $CommBank (CBA.AU)$ third-quarter update is also weighing on Westpac today.

Investors tend to use information from other companies in the same industry to shape expectations. If CBA's update is anything to go by, the Australian banking sector might be cooling off.

Australia's largest bank experienced a 5% fall in unaudited statutory net profit after tax in Q3, easing to $2.3 billion. The weaker bottom line resulted from a 1% slip in operating income and a 2% increase in operating expenses.

The big four banks are all in the red day following the news. Westpac shares, though, are the worst of the bunch.