"On the threshold" how is it written?
On the eve of something - in this case "in" is a pretext (in anticipation of something), "before" is a prefix, "door" is the root. Accordingly, "on the threshold" is written with two letters "d".
"On the threshold" can be used as aa pretext indicating an event that is expected in the near future. The main meaning of this word is "before the door, before something" and the second meaning, portable, is the "beginning of something".
"On the threshold": How is this word written in the meaning of the beginning of something? "On the eve of the bathing season" - that is, at the beginning of the swimming season. In this case, we are dealing with a preposition, and this word has a portable meaning, so it is written with "and" at the end.
This word can also be written with the letter "e" onend if it matters. For example, "on the threshold of the temple," that is, before entering the temple. In this case, the word has the first value, we write with "e" at the end.
Thus, the choice of the right variant "on the eve of" or "on the threshold" depends on the meaning, therefore it is necessary to choose one or another spelling, proceeding from the context.