The special election to fill John Kerry’s seat in the U.S. Senate will be in late June, after most schools have let out for the summer and vacation season begins to heat up, Secretary of State William Galvin said on Monday. Gov. Deval Patrick plans to appoint an interim senator on Wednesday.

Anticipating that Kerry will resign Tuesday after the full Senate confirms his nomination to become secretary of state, Gov. Deval Patrick has chosen June 25 as the date for the special election to fill the remainder of his term through 2014, Galvin said, adding that he anticipates Democratic and Republican primaries will be held April 30.

Galvin said he was told by the secretary of the United States Senate that Kerry plans on Tuesday to submit his resignation, effective on Friday, triggering the state law governing Senate vacancies.

State law mandates that the special election be held within 145 to 160 days of a vacancy, created by the submission of a resignation or death. The late June date settled on by Patrick is one of the earliest he could have picked, coming 147 days from Tuesday and falling just before the July 4 holiday.

After interviewing Kerry during a confirmation hearing last week, the Senate’s Foreign Relations Committee has scheduled a vote for Tuesday to refer Kerry’s nomination to the full Senate, and Senate Majority Leader Harry Reid has indicated the Senate will follow up with a confirmation vote the same day.

If that schedule holds, Patrick told reporters in East Boston on Monday that he will announce his selection to fill Kerry’s seat on an interim basis Wednesday. Asked whether he had made a final decision, Patrick said, “Pretty much. I have one or two other questions to ask. We’ll be ready.”

The governor has kept his internal list of candidates confidential, with the exception of former Congressman Barney Frank who has publicly talked about wanting appointment and talking to the governor about it.

Asked if the public would be surprised by the pick, Patrick on Monday said, “I told you we’re going to have someone that I am convinced will be a wise steward of the interest of the people of the Commonwealth while we wait for the people to elect a senator in a special election. And I continue to believe that the main event is the special election.”

Galvin said he had originally recommended that the special election be held on June 18 because school buildings often used as polling locations in municipalities would still be open. Patrick, however, didn’t think it was appropriate to tweak the current law to hold an election earlier than 145 days, according to Galvin.

Galvin said he would have preferred an earlier date to hold voter interest, but said the law and the timing of the U.S. Senate’s consideration of Kerry’s nomination prevented that. He said he supports Patrick’s decision.

President Obama nominated Kerry Dec. 21 and while his confirmation has never seemed in doubt, the process has taken more than a month. Galvin said Kerry ideally could have been confirmed in December or early January, but said the Senate wanted Secretary of State Hillary Clinton to testify about the consulate attack in Benghazi before she resigned, and illnesses delayed her appearance before Congress.

If Patrick signs the order setting the election date by Wednesday, Galvin said he is prepared to immediately release nomination papers, giving candidates four weeks to collect the 10,000 signatures required to have their named included on the ballot. He has also filed legislation to allow cities and towns with municipal elections scheduled this spring to circumvent local bylaws and match up their local elections with the Senate primary.

Galvin said the matter is time sensitive because cities and towns may soon be releasing nomination papers for local elections, or in case like Cohasset, have bylaws requiring 90-day notice to change the date of an election.

“We do need some legislation and we don’t see it as controversial. We see it as purely technical and also practical because it’s going to save money,” Galvin said.

U.S. Rep. Edward Markey is so far the only candidate from either party to formally declare his intention to run for Kerry’s seat, though U.S. Rep. Stephen Lynch is also weighing a campaign, and could announce before the end of the week whether he wants to challenge Markey for the nomination.

Other Democrats, such as U.S. Rep. Michael Capuano and state Sen. Benjamin Downing, considered running, but decided it against it as the Democratic establishment began to coalesce behind Markey.

On the Republican side, former U.S. Sen. Scott Brown has been quiet about his intentions, and most in the party are waiting to see what he does before potentially entering race.

Markey on Monday challenge any prospective opponents from either party to commit to the “People’s Pledge,” a reference to the agreement reached between Brown and Sen. Elizabeth Warren during their 2012 race to keep third-party money out of the campaign.

“If all the candidates agree, we can give the voters the kind of debate they deserve. This election should be a forum for the voices of everyday voters, not attacks from Karl Rove and other outside special interests. This election should be focused on big issues and ideas, not big-money outside groups. I urge all candidates to join me in ensuring that Massachusetts once again will be the leader for the nation on this issue,” Markey said in a statement.

Lynch, according to a spokesman, said on Jan. 10 that he would also support a similar pledge if he decides to run.

The agreement between Brown and Warren, enforced with penalties on the campaigns if outside groups spent money on their behalf, largely held up but some have questioned whether it could be as effective in a shorter, five-month special election that is one of the only races going on in the country.

"I love the idea of not being awash in money," Patrick said earlier this month, adding, "I just don't know the puts and takes of how that sort of thing works in a four or five month special election.”