By verse John 1:19, John has been baptizing some 6 months before the time Jesus comes to him. The elders finally take notice and arrive to ask John who he is. John says I am not the Christ, but the voice of one crying in the wilderness as Isaiah prophesied. He basically has gone public; he has stood.
John 1:29 the next day, after the interrogation, Jesus comes to him and is baptized. John testifies that Jesus is the Christ. Christ's baptism is day one.
Day 1, baptism
John 1:35 then says “again the next day” and two disciples follow Jesus. Is this a new day or is it later the same day?
Again the next day after John stood, and two of his disciples;
There are three keys to understand.
One key is the word “again”. It is palin. This is what it means.
Strong’s: πάλιν pálin, pal'-in; probably from the same as G3823 (through the idea of oscillatory repetition); (adverbially) anew, i.e. (of place) back, (of time) once more, or (conjunctionally) furthermore or on the other hand:—again.
Vine’s: Again: the regular word for "again," is used chiefly in two senses,
(a) with reference to repeated action;
(b) rhetorically, in the sense of "moreover" or "further," indicating a statement to be added in the course of an argument,
In other words, by including the word “again”, John is telling us that two things happened on the same day. Christ was baptized and Andrew and Peter became disciples.
Furthermore, by indicating it was about the tenth hour (4-5pm), it tells us it was on the same day, else why even mention the late hour?
Lastly, the verse says John stood. What does that mean?
Strong’s †ἵστημι hístēmi, his'-tay-mee; a prolonged form of a primary στάω stáō stah'-o (of the same meaning, and used for it in certain tenses); to stand (transitively or intransitively), used in various applications (literally or figuratively):—abide, appoint, bring, continue, covenant, establish, hold up, lay, present, set (up), stanch, stand (by, forth, still, up).
The tense is pluperfect. It means “In Greek occurs rarely. It corresponds in a single Greek word to the sense of the English pluperfect, which indicates an event viewed as having been once and for all accomplished in past time.”
The idea is when John had proclaimed once and for all publicly who he was on the day before that he was standing, he was appointed, he was established.
Day zero, John says he is the one prophesied.
Day 1, baptism, two recorded disciples
John 1:43 the day following at least two more people become Jesus’ disciples.
Day 2, more disciples
John 2:1 on day three, there is a wedding in Cana.
Day 3, wedding
So, the three days are in relation to Jesus as follows.
Day 1 baptism and disciples
Day 2 more disciples
Day 3 wedding at Cana