The word rejoice is listed 11 times in the Answer and Covenant, including the previous revelations that were part of the A&C.
All 11 listed below: (parenthesis mine for context)
1. There is great reason to rejoice because of the work that has been done. (Current day work on the restoration edition of the scriptures). Pg 1.
2. Behold, your sins are forgiven you; you are clean before me; therefore, lift up
your heads and rejoice. (Revelation to Joseph Smith and Oliver Cowdry) Pg 4
3&4. Let the hearts of your brethren also rejoice, and let
the hearts of all my people rejoice who have, with their might, built this
house to my name. For behold, I have accepted this house, and my name
shall be here; and I will manifest myself to my people in mercy in this house. (Revelation to Joseph, speaking about the Kirtland Temple) Pg 4
5. Behold and see: the hearts of thousands and tens of thousands shall greatly
rejoice in consequence of the blessings that shall be poured out. (Same revelation to Joseph and Oliver) Pg 4.
6&7. Rejoice in
me, and rejoice with your brethren and sisters who are mine also. Pg 7
8. Then by my law and my word they will be mine and I will be with and lead my people
onward through the Spirit of Truth, the Comforter, the Record of Heaven, the peaceable
things of immortal glory, even the Holy Ghost which will abide with them, and you will be
children of the Most High God, fellow servants and numbered with the congregation of the
just. Therefore rejoice! Pg 9
9&10. But if righteousness returns and my people prove
by their actions, words and thoughts to yield to my Spirit and hearken to my
commandments, then will the Earth rejoice, for the feet of those who cry peace upon her
mountains are beautiful indeed, and I, the Lord, will bring again Zion, and the earth will
rejoice. Pg 9
11. Be comforted, be of good cheer, rejoice, and look up, for I am with you who remember me,
and all those who watch for me, always, even unto the end. Amen. Pg 10
On the same note, the word joy appears in the A&C as follows:
If you return good for evil you will cleanse yourself and know the joy of your
Master. Pg 7.
So what reason do we have for all these statements about rejoicing? God is a God of Joy. He wants people to rejoice and have joy. Yet I admit I don't really understand how to rejoice. When I look around me at the events of 2017 and even among those who attempt to be God's people there does not appear to be any great reason to rejoice. They say it's difficult to see and understand events as you are living them. They become more clear with hindsight.
I personally struggle at times with what rejoicing means our ought to look like. Growing up in the LDS church external demonstrations of rejoicing, or expressions of joy and delight are not common. And the rejoicing often seen in other churches is something my upbringing taught me was inappropriate or weird or not suitable for the solemn, dignified, and sober, attitude I was always supposed to have. Nothing I hear these days during a typical 3 hour block of LDS meetings causes me to do anything other than mourn. Let alone rejoice.
The times when I have felt the joy and rejoicing the scriptures speak of were moments of closeness to the Lord. Moments when His love and His light were around me and in me. Those indeed caused my heart to rejoice. Often I could find not adequate external expression, nor were words adequate. But those moments for me have not always been permanent. Life returns to normal and there are bills to pay, kids to raise, and challenges to overcome.
But what I have learned is that our Lord is joyful. So I seek joy, because I seek the Lord and His righteousness. And the Lord says to rejoice. Because of Him, we can.