The day of the Lord will come like a thief in the night (1 Thes 5:2).
People sleep during the night or stay up getting drunk. Either way they are not alert (v.7).
Christians live in the daytime (v.4), even though for the world it is still dark. They are experiencing the already even though it is not-yet. Therefore, they should be alert for Christ's appearing and doing the things of the day. That doesn't mean having your head in the clouds and being idle, but working (v.12-14)
Tomorrow's Monday, so unglamorous as it sounds, I'll be at the office as a child of the day. Today was the Lord's day, but so is Monday.