Matt 24:36 begins with “No one knows about that day or hour …” and, if I’m not wrong, this passage refers to Jesus Christ’s second coming, but this second coming happens exactly 1260 after the abomination of desolation and so it looks like it is precisely predictable. If that is true, could that sentence in any way be referable to the rapture, event of which we cannot know the day nor the hour?”
This is a common misunderstanding. The Bible does not say the 2nd coming will take place exactly 1260 days after the abomination of desolation. It only says the Great Tribulation will last for 1260 days. And in Matt. 24:36 Jesus only said no one knows about that day or hour. He was referring to the details concerning the 2nd Coming. As for the actual timing of the 2nd Coming, in Matt. 24:42 & 44, Matt. 24:50, and Matt. 25:13 the Lord said no one on earth at the time will know the exact day or hour of His return.
I believe this will be partly due to the fact that immediately after the end of the Great Tribulation both the sun and the moon will go dark, making it impossible to confirm the passage of time (Matt. 24:29).