SALT LAKE CITY — The LDS Church mailed 1,150 letters extending mission calls to young Mormon men and women on Tuesday over the signature of President Russell M. Nelson, a symbol of the transition underway in church leadership.

The Quorum of the Twelve Apostles is leading the church during what is known formally as an apostolic interregnum, with President Nelson leading the quorum as the senior apostle.

“Even when there’s an impending change in church leadership, the work of the church moves forward," President Nelson said in a news release Tuesday. "We don’t want to have our prospective missionaries wait for their calling any longer than is necessary for this much-anticipated milestone in their lives."

Mission calls bore the signature of President Thomas S. Monson from February 2008, when he became the 16th president of The Church of Jesus Christ of Latter-day Saints, until his death Jan. 2 at age 90.

President Monson’s signature appeared on mission calls to 410,442 full-time missionaries, roughly three of every 10 missionaries who have served since the church was organized in 1830.

Today, nearly 67,000 Mormon missionaries serve in 421 missions around the globe.

The call to serve as an apostle is a lifetime position. When the president of the church passes away, the First Presidency is dissolved immediately and the power or “keys” to lead the church transfer to the Quorum of the Twelve, which is led by the senior apostle.

"This time of transition in the leadership of the church is a rare and marvelous moment," Elder Jeffrey R. Holland of the Quorum of the Twelve said Tuesday. "It is, of course, bittersweet to say goodbye to one prophet and lovingly welcome another.

"It is a dramatic time for the revelation of the Lord to come to a unique body of men. The organization of the presiding quorums of the church is not something any business school would teach. It is not like any organizational chart you have ever seen in your life — threes and twelves and seventies — all kinds of unusual numbers.

"But when it comes time to say goodbye to one prophet and anticipate a new one, that organization guarantees, through all the drama of it and through all the anticipation of it, that there is never a moment when the divine apostolic keys are not in force. And with those keys in place, there is never any lack of revelatory experience. It is a little bit like the British (saying in the same breath), 'The king is dead, God save the king.' The transition is that fast, because, of course, the authority to lead is already in place."

President Nelson, as the next senior apostle to President Monson, has served as president of the Quorum of the Twelve Apostles for nearly 30 months.

By precedent, the senior apostle has become the next church president after a short interim, usually about a week but ranging from four to 11 days.

That wasn’t always the case. In the early church, interims lasted 21-42 months.

"They were much lengthier in the early days of the church with a lack of precedence and other issues," said Brandon Metcalf, an archivist at the LDS Church History Library.

But in 1898, President Lorenzo Snow was set apart as the new president of the church 11 days after the death of President Wilford Woodruff.

That was President Woodruff’s intent, Metcalf said.

First, there is an account of President Woodruff receiving a prompting in the temple to not delay reorganizing the First Presidency. Second, President Woodruff counseled then senior Apostle Snow to do the same in a private conversation six years before his death.

"When I go, I want you, Brother Snow, to not delay the organization of the First Presidency," President Woodruff said, according to an account President Snow dictated.

"Wilford Woodruff felt very strongly after his experience of nearly two years of an apostolic presidency in admonishing Lorenzo Snow not to delay after his funeral," Metcalf said.

President Monson’s funeral is Friday at noon at the Conference Center next to Temple Square. The current period of apostolic presidency is likely to be the longest interregnum since that 1898 transition from President Woodruff to President Snow.

If the next church president is not set apart by Sunday, the current period will become the longest interregnum since the 20-month transition from President John Taylor to President Woodruff, which ended in 1889.

Elder M. Russell Ballard, the third most senior apostle in the church, said quorum business is relatively light this week.

"It's been very quiet as we honor the passing of a president," he said. "In reverence to the deceased president, we just don't do much prior to the funeral. There's not much to talk about, really, not much you can say or do or decide because there will be a new First Presidency, and then when the new First Presidency is in place, then we will be very busy, very busy."

Elder Ballard said the work of the Twelve is focused on helping church members.

"What is the most important thing we can do to help members of the church get the testimonies of the gospel from their minds down into their hearts so that they can weather whatsoever is going on out in this crazy world? The world is a lot tougher world than I think anything I've ever experienced, particularly the moral issues, the uncertainty. Social media has run away with the world. The cellphone is in control. A lot of different things. It requires the leadership of the church to think differently about how you get the message into the hearts of our people."

The quorum will meet soon after President Monson's funeral to reorganize the First Presidency. Then other business will resume full speed under the direction of a new president.

"It's a transition," Elder Ballard said, "and I would think by next week this time we'll all be wondering how we're going to get it all done."