Genesis 1:3 And God said, “Let there be light,”a and there was light. 4And God saw that the light was good, and He separated the light from the darkness. 5God called the light “day,” and the darkness He called “night.” And there was evening, and there was morning—the first day.
The first day is the beginning of light and physical time. The last day is the end of physical time.
The Bible talks about "last days" (plural) with multiple meanings:
Gen 49:1 And Jacob called unto his sons, and said, Gather yourselves together, that I may tell you that which shall befall you in the last days.
Isa 2:2 And it shall come to pass in the last days, that the mountain of the LORD'S house shall be established in the top of the mountains, and shall be exalted above the hills; and all nations shall flow unto it.
Act 2:17 And it shall come to pass in the last days, saith God, I will pour out of my Spirit upon all flesh: and your sons and your daughters shall prophesy, and your young men shall see visions, and your old men shall dream dreams:
2Pe 3:3 Knowing this first, that there shall come in the last days scoffers, walking after their own lusts
The Bible speaks of a future last day (singular) with only one meaning.
Jhn 11:24 Martha saith unto him, I know that he shall rise again in the resurrection at the last day
Jhn 12:48 He that rejecteth me, and receiveth not my words, hath one that judgeth him: the word that I have spoken, the same shall judge him in the last day.
The last day is the day of resurrection and the end of physical time.
John 5:28 “Do not be amazed at this, for a time is coming when all who are in their graves will hear his voice 29and come out—those who have done what is good will rise to live, and those who have done what is evil will rise to be condemned.
For us, the last day is the end of physical time and the beginning of a new experience of eternal time.