I don't believe either phrase refers to "a singular 24-hr day," thus...
"the Day of the Lord" (a period-of-time-[not merely 24-hrs]-of-JUDGMENTs followed by a period-of-time-[also not merely 24-hrs]-of-BLESSINGs [ALL earthly-located]--all of that together makes up "the Day of the Lord") is:
--[DOTL] something distinct from the Subject of v.1 (which v.1 speaks solely of the Subject of our Rapture)
--[DOTL] something Paul was cautioning them not to be persuaded by false-conveyors "IS PRESENT" (and would indeed be "troubling" to them, because it starts out with the "JUDGMENTs" time-period aspect), v.2 so "don't believe THEM"... and saying also, "believe what WE taught you INSTEAD" (v.15)
--[DOTL] something that would be REASONABLE for them to have been persuaded "IS PRESENT [PERFECT indicative]," based on their PRESENT and ONGOING, VERY NEGATIVE experiences they were ENDURing (2Th1:4)
--[DOTL] something that Paul was telling how would "FIT" [time-wise] IN RELATION TO the other thing (or, the SEQUENCE between the one and the other--this is why v.1's Subject is distinct from v.2's Subject--how they "fit" together TIME-WISE/SEQUENCE-WISE)
--[DOTL] something they already "KNOW PERFECTLY" would ARRIVE like "a thief IN THE NIGHT" and like the INITIAL "birth PANG [SINGULAR]" (matching what Jesus spoke of in the Olivet Discourse, Matt25/Mk13... THAT would indeed FREAK THEM OUT to think had ARRIVED and "IS PRESENT")...
... so they understood "WHAT" that is [involving an earthly time-period OF JUDGMENTs]...
... what they needed was a letter (which would have taken SOME TIME to reach them) telling them not to be convinced by anyone persuading them it "IS PRESENT" (again, their PRESENT and ONGOING "tribulations and persecutions would have made this PERFECTLY REASONABLE for them to believe [the DOTL] "IS PRESENT" [tho it wasn't])... It makes NO SENSE that they would have been convinced by someone telling them "[they] MISSED THE RAPTURE"... No! (this is NOT indicated by the text). They were under no such delusion, nor were they at risk of being under any such delusion (all one has to do is "look around" to see with one's eyes that this is not so... and these were not dense people ... and what kind of LIAR would think such a thing would be to any degree persuasive to them, considering such tangible evidence abounding?? )
"the Day of the Lord" (a period-of-time-[not merely 24-hrs]-of-JUDGMENTs followed by a period-of-time-[also not merely 24-hrs]-of-BLESSINGs [ALL earthly-located]--all of that together makes up "the Day of the Lord") is:
--[DOTL] something distinct from the Subject of v.1 (which v.1 speaks solely of the Subject of our Rapture)
--[DOTL] something Paul was cautioning them not to be persuaded by false-conveyors "IS PRESENT" (and would indeed be "troubling" to them, because it starts out with the "JUDGMENTs" time-period aspect), v.2 so "don't believe THEM"... and saying also, "believe what WE taught you INSTEAD" (v.15)
--[DOTL] something that would be REASONABLE for them to have been persuaded "IS PRESENT [PERFECT indicative]," based on their PRESENT and ONGOING, VERY NEGATIVE experiences they were ENDURing (2Th1:4)
--[DOTL] something that Paul was telling how would "FIT" [time-wise] IN RELATION TO the other thing (or, the SEQUENCE between the one and the other--this is why v.1's Subject is distinct from v.2's Subject--how they "fit" together TIME-WISE/SEQUENCE-WISE)
--[DOTL] something they already "KNOW PERFECTLY" would ARRIVE like "a thief IN THE NIGHT" and like the INITIAL "birth PANG [SINGULAR]" (matching what Jesus spoke of in the Olivet Discourse, Matt25/Mk13... THAT would indeed FREAK THEM OUT to think had ARRIVED and "IS PRESENT")...
... so they understood "WHAT" that is [involving an earthly time-period OF JUDGMENTs]...
... what they needed was a letter (which would have taken SOME TIME to reach them) telling them not to be convinced by anyone persuading them it "IS PRESENT" (again, their PRESENT and ONGOING "tribulations and persecutions would have made this PERFECTLY REASONABLE for them to believe [the DOTL] "IS PRESENT" [tho it wasn't])... It makes NO SENSE that they would have been convinced by someone telling them "[they] MISSED THE RAPTURE"... No! (this is NOT indicated by the text). They were under no such delusion, nor were they at risk of being under any such delusion (all one has to do is "look around" to see with one's eyes that this is not so... and these were not dense people ... and what kind of LIAR would think such a thing would be to any degree persuasive to them, considering such tangible evidence abounding?? )
Then, let us suppose that "Day of the LORD" is correct. It's length of time is of no matter, for the "gathering" of the saints probably takes less than five minutes. Next, a "thief" comes quietly and leaves quietly. But other scriptures give the "day of the LORD" as tumultuous. Next, a thief comes only for what is valuable. The judgment of the ungodly shows their value - none. Next, the usage of the Lord's coming as a "thief" in Matthew 24 is connected with Him TAKING somebody. The Greek word used is "paralambano" which means, "to take someone well-known with on a journey" - like Joseph "took" Mary with to Egypt (Matt.2:13). Besides all these, it is also out of place, for the "day of the Lord" is a day of vengeance - not the joyous "gathering unto Him".
Would you not agree that all the above problems fall away if the "day of Christ" was correct. The sequence would then be;
- The Church apostatizes itself and have no interest in Christ
- The Overcomers look for Christ's appearing and presence in the clouds
- The Beast is revealed as a charismatic and capable leader but lawless
- The Overcomers are "gathered" suddenly, without notice, for the "Thief" came and went silently - THE DAY OF CHRIST
- The Beast is crowned and starts the last 7 years by making it possible for Israel to have their Temple and oblations
- Lawlessness abounds
- The kings that invested their power in the Beast end all formal religion and direct worship at the Beast
- The Beast sets an idol of himself in the Holy of Holies of Jerusalem's temple
- The Great Tribulation starts with this Abomination of Desolation - THE DAY OF THE LORD BEGINS
- The Beast requires the Mark on all men
- The Two Witnesses warn the Nations and Israel of this breach with God
- After the death, resurrection and rapture of the Two Witnesses, an angel proclaims the eternal gospel from mid-heaven
- Men give glory to God in defiance of the Beast after the resurrection and rapture of the Two Witnesses
- The Beast gathers his armies to subdue Jerusalem
- The Lord descends, NOT as a "THIEF", but as "LIGHTENING" and saves Jerusalem
- The Lord proceeds to Jezreel and meets and defeats the armies of the Beast
- The DAY OF THE LORD starts at point # 9 and the earth reels under the despotic rule of the Beast and the wrath of God