Stimulus Day — not to mention St. Patrick — is a reason many will raise a glass on Wednesday.

A third coronavirus stimulus check officially landed in millions of Americans’ bank accounts on Wednesday morning, according to the feds.

The Internal Revenue Service said it started processing that first batch of $1,400 payments — worth more than $242 billion — last week, but the funds didn’t actually become available until the official payment date of March 17.

While most of those payments were sent via direct deposit, they also include about 150,000 paper checks worth some $442 million that the US Treasury has mailed, officials said.

The IRS hasn’t provided a detailed timeline for distributing the rest of the checks, but President Biden said Monday that his administration would reach the milestone of delivering 100 million payments “over the next 10 days.”

While the feds trumpeted the start of the so-called economic impact payments on Friday, many Americans wondered why they had to wait several days for the money to arrive.

The answer lies in the machinery of the financial system. The IRS sent the payments through the Automated Clearing House system, an electronic network that’s used to process financial transactions.

Each deposit in the first wave had a settlement date of March 17, meaning that’s when the money would actually move from the IRS’ coffers to banks, which then make it available to their eligible customers, according to the National Automated Clearing House Association, which manages the ACH system in the US.

To get more specific, the group known as Nacha said the payments will be settled at 8:30 a.m. Eastern time Wednesday, “exactly as instructed by the IRS.”

“This is literally the moment in time when the money will be transferred from the government to banks’ and credit unions’ settlement accounts at the Federal Reserve,” Nacha said in a statement.

Banks and credit unions are then required to make the money available to account holders by 9 a.m. local time, according to the group.

Some financial institutions such as the banking startup Chime made the funds available early. The IRS also said some people would see the money in their accounts as provisional or pending payments before Wednesday.

Megabanks such as JPMorgan Chase and Wells Fargo, however, told customers that the checks wouldn’t be available until Wednesday at the earliest, leading to some online criticism that they were sitting on the money.

But industry groups said banks did not actually have any of the money before the official payment date.

“Until that time, the funds remain with the government,” the American Bankers Association and eight other trade groups said in a statement. “While the IRS could have chosen to send the funds via Same Day ACH or provided for an earlier effective date, it chose not to do so.”