Where is the Bible verse that mentions “today is the day of salvation”?

I think you’re probably thinking of 2 Corinthians 6:2

For he says,
“In a favorable time I listened to you,
and in a day of salvation I have helped you.”

Behold, now is the favorable time; behold, now is the day of salvation.

The first part of the verse is a quotation from Isaiah 49:8.

A useful way to find passages when you can remember a few words is to search at Bible Gateway.

For example, a search for “day of salvation” gives these results.