This probably refers to the day when Christ returns. Paul often connected the return of Christ with motivating godly living. (See: [[rc://en/tw/dict/bible/kt/godly]])
A paradox is a true statement that appears to describe something impossible. This statement in verse 21 is a paradox: "to die is gain." In verse 23 Paul explains why this is true. ([Philippians 1:21](../../php/01/21.md))