The very word salvation implies that we are saved from something. It's a very important word in the Bible. It is used 120 times in the NIV Bible, 158 times in the KJV.
The Hebrew word for salvation used here is yĕshuw`ah and means:
a) welfare, prosperity
c) salvation (by God)
In the Old Testament, the Psalmist rejoiced in the deliverance, welfare, and victory the Lord had been so loving to give him. God led the Psalmist, and many more to be sure, to victory and even righteousness obtained through faith, as we learn in Hebrews 11.
The Psalmist mourned greatly over the sin that was before him. Regarding his sinful ways he said:
The Hebrew word for mourning is qadar. This verb means:
a) (Qal) to be dark
1) to darken
2) to cause to mourn
c) (Hithpael) to grow dark
It is in this mourning that he found joy in the salvation that came from the Lord, I believe. He knew the Lord provided salvation to all who call on His Holy Name.
"Godly sorrow brings repentance that leads to salvation and leaves no regret, but worldly sorrow brings death." ~ 2 Corinthians 7:10 (NIV)
"Blessed are those who mourn, for they will be comforted." ~ Matthew 5:4 (NIV)
1) to call to one's side, call for, summon
2) to address, speak to, (call to, call upon), which may be done in the way of exhortation, entreaty, comfort, instruction, etc.
a) to admonish, exhort
b) to beg, entreat, beseech
1) to strive to appease by entreaty
c) to console, to encourage and strengthen by consolation, to comfort
1) to receive consolation, be comforted
d) to encourage, strengthen
e) exhorting and comforting and encouraging
f) to instruct, teach
We have peace and joy that comes from knowing we have been reconciled to Christ. In mourning our past sins we find the how deep God's grace and mercy for us are, but we need not keep mourning. We have been reconciled, redeemed, renewed.
Today is the day to rejoice in the Lord's salvation, my friend! How can we not? Our debt for our sin was paid by our Savior and we may be sure that He wants us to rejoice.
2) to rejoice exceedingly
3) to be well, thrive
4) in salutations, hail!
5) at the beginning of letters: to give one greeting, salute