The prophet who emphasized the concept of "the day of the Lord" as a time of judgment and salvation was

The prophet who emphasized the concept of "the day of the Lord" as a time of judgment and salvation was Joel. In the book of Joel, he speaks about the day of the Lord as a time when God will intervene in human history to judge the wicked and bring salvation to his people.
Joel describes the day of the Lord as a day of darkness and destruction, when the Lord will bring judgment upon the nations for their wickedness. He warns the people to repent and turn to the Lord, so that they may be saved from the coming judgment.
But Joel also brings a message of hope and restoration. He tells the people that if they repent and return to the Lord with all their hearts, he will show them mercy and pour out his Spirit upon them. This outpouring of the Spirit will bring about a time of blessing and abundance, when the Lord will restore what has been lost and guide his people in righteousness.
So, in summary, Joel emphasized the concept of the day of the Lord as a time of judgment for the wicked and salvation for those who repent and turn to the Lord. He encouraged the people to seek the Lord and follow his ways, so that they may be saved and experience the blessings of his kingdom.