John Gill Commentary Psalms 30:11

"Thou hast turned for me my mourning into dancing; Thou hast loosed my sackcloth, and girded me with gladness;" — Psalms 30:11 (ASV)

You have turned for me my mourning into dancing
This, with what follows, expresses the success he had in seeking the Lord by prayer and supplication; there was a sudden change of things, as it often is with the people of God; sometimes they are mourning by reason of sin, their own and others; or on account of afflictions; or because of spiritual decays; or through the temptations of Satan; or, as it was the case of the psalmist now, because of the hidings of God's face.

But this mourning is exchanged for joy and gladness when the Lord discovers his pardoning love, revives his work in their souls, takes off his afflicting hand from them, rebukes the tempter, and delivers out of his temptations, and shows himself, his grace and favour.

you have put off my sackcloth ;
which was used in mourning for relations, and in times of calamity and distress, and as a token of humiliation and repentance, (Genesis 37:34) (Esther 4:1) (Jonah 3:8) ;

and girded me with gladness ;
by these phrases the same thing is signified as before; see (Isaiah 61:3) .