Summary: How Death and the Day of the Lord are one and the same.
The last of the 66 books in the Bible, the Revelation, speaks about the end of the world and Judgment Day where humans have to do their reckoning, after which they will go to Paradise or Hell. But when was this supposed to happen? Jesus himself once said "Verily I say unto you, There be some standing here, which shall not taste of death, till they see the Son of man coming in his kingdom" (Matthew 16:28).
In the Revelation of St. John this concrete deadline (pun intended) is replaced by "shortly":
As the years passed by, purgatory was invented - where the sinners are staying "Till the foul crimes done in [their] days of nature are burnt and purg'd away". This meant that judgement would happen for the individual person at the time of death - or as is written in the Bible (Romans, 14,12): "So then every one of us shall give account of himself to God".
In reality, the difference was less than clear: For the individual who fell prey to the plague, cholera, the 100-years war, famine etc., death was the start of the personal reckoning but at the same time these catastrophes were regarded as forebodings of the coming apocalypse and the great collective reckoning.
The result was that the day of reckoning was no longer a great collective event that would fit into the great scheme, but was now a very personal experience that might arrive at a very inconvenient time - maybe without giving you time to pay penance for all of your sins.
It's a recurring theme in Lübeck's dance of death, that the humans complain that Death is coming unexpectedly and inconveniently. This is particularly true for the merchant:
But when was this going to happen? We read in 2nd Peter 3:4 that people grew impatient(1) and asked "[...] Where is the promise of his coming? for since the fathers fell asleep, all things continue as they were from the beginning of the creation". Peter (who hadn't been a fisher for nothing) gave them a wonderful explain-all, namely "[...] that one day is with the Lord as a thousand years, and a thousand years as one day." (v8) and then he added ominously (v10): "But the day of the Lord will come as a thief in the night(2);[...]"
So Death and the day of the Lord was the same and both would come unexpectedly. In Lübeck's dance of death, Death tells the youth:
Death is quoting Job 24:14: "The murderer rising with the light killeth the poor and needy, and in the night is as a thief". The English Bible uses a different wording than the German Bibles: in Luther's Bible the phrase is "des Nachts schleicht der Dieb" (in the night the thief sneaks) and in Elberfelder "in der Nacht geht der Dieb um" (in the night the thief goes around). In Lübeck, Death combines them and says "Slikende is min Ummewank" (literally: sneaking is my around-going).
Death as a sneaking thief appears in all the dance of death manuscripts:
Wuste de huswerd to welker tyd
If the goodman knew what time
De doet sendet jw neenen breff
Death does not send you any letter
Viste hosbonden i huilcken tiid
If the goodman knew what time
This time the Bible(3)quotes are really lining up:
Notice that the youth had himself just used he word "sneaking":
First the youth complains about the bad timing (compare with the top of this page) and then says literally "you come sneaking here going". This may explain why Death feels like showing how well versed he is in the Bible - in front of the youth who probably couldn't care less.
Footnotes: (1) (2) (3)