"11. The trup symbol of psik alludes to the day that Moshe added at his own initiative.
In the verse (Shemos 19:11) והיו נכונים ליום השלישי, כי ביום השלישי ירד ה' לעיני כל העם על הר סיני, there is a trup symbol of psik (a vertical bar | ) after the word כי, and before the words ביום השלישי. And it is possible to say that this alludes to that which is stated in the sugya regarding Mattan Torah in Maseches Shabbos (daf 87a), that they ask from this verse, that it is written והיו נכונים ליום השלישי, upon Rabbi Yossi who maintains that on Shabbos, which was the seventh of Nissan, the Torah was given. And they answer that behold we say that Moshe added a day of his own initiative, even as in the commandment in this verse is stated that it should be on the third day from now, see inside in the gemara.
And it seems possible to say that the trup symbol of psik hints to this, that in actual fact the commanded time was extended and a day was added to it, and only on the third day of complete days, with the night of the day with it, did Hashem descend to the eyes of the entire nation upon Har Sinai."
The gemara in question is this:
Come and hear: And be ready against the third day: this is a difficulty according to R. Jose?15 — Surely we have said that Moses added one day of his own understanding!Though this is not the primary verse from which the gemara asks. Rather, this is the stama degemara extended what was already stated earlier on the daf:
On Tuesday he said to them, and ye shall be unto me a kingdom of priests; on Wednesday he informed them of the order to set boundaries, and on Thursday they separated themselves. An objection is raised: And sanctify them to-day and to-morrow:3 this is difficult in the view of R. Jose?4 — R. Jose can answer you: Moses added one day of his own understanding.5 For it was taught, Three things did Moses do of his own understanding, and the Holy One, blessed be He, gave His approval:6 he added one day of his own understanding, he separated himself from his wife,7 and he broke the Tables. 'He added one day of his own understanding': what [verse] did he interpret? To-day and to-morrow: 'to-day' [must be] like 'tomorrow: just as to-morrow includes the [previous] night, so 'to-day' [must] include the [previous] night, but the night of to-day has already passed! Hence it must be two days exclusive of to-day.This also forms the primary derivation, as opposed to this secondary remez / allusion.
Here is an image of the trup of the pasuk:
As we might already expect, this is not really a psik, but is rather a munach legarmeih. Note that it is before the short word כי, and note the munach revii following. This is a regular disjunctive accent, caused by syntax and mechanical considerations such as pasuk length and tree structure, rather than some semantic reason such as to hint that the three days were extended.