The basic word for a day — whether a literal 24-hour period, the daylight portion (as opposed to night), or a broader era/period ('in the days of Herod,' 'the day of the Lord'). It appears over 380 times in the NT, structuring narratives ('on the third day'), marking eras ('the last days'), and pointing to eschatological culmination ('that day').
The 'day of the Lord' (hēmera kyriou) is one of the NT's most powerful eschatological concepts, inherited from the OT prophets (Joel 2:31; Amos 5:18). It denotes God's decisive intervention — judgment for the wicked, deliverance for the righteous. Jesus spoke of 'that day' when He would return (Matt 24:36; Mark 13:32). Paul urged believers to live 'as children of the day' (1 Thess 5:5-8), contrasting the moral illumination of faith with the darkness of sin. 'The third day' became shorthand for resurrection — Christ was raised 'on the third day' (1 Cor 15:4), fulfilling the Jonah sign and the temple prophecy. Every Lord's Day (Sunday) thus became a weekly 'day of resurrection,' reorienting time itself around the risen Christ.