Background: Exegetical tips
Before "harmonizing", I want to point out some exegetical tips to help you read these books.
- Because something is omitted in a work does not mean the work asserts it didn't happen. A lot of questions on this board are of this form, and it's counterproductive. These are highly condensed summaries of events where every word is carefully chosen to make a certain point, rather than comprehensive "all the news that's fit to print" timelines of historical events.
You also see the same issue come up in the gospels, where say Luke eloborates more on an event than Mark which generates all sorts of confusion as to whether Luke "contradicts" Mark. That is not something that requires any more "harmonization" than to observe some details were not included in Mark, but were included in Luke.
Prophecy should be viewed as a set of images organized thematically rather than chronologically. E.g. just because A is described before B in the prophetic work does not mean that the historical event corresponding to A happened before the historical event corresponding to B.
Prophecy, as messages from God, should be read as God's view of an event, rather than man's view. Therefore prophecies sometimes include explanations for why or how something happened that don't correspond to the explanations viewed by participants in the event. For example in prophecies you see God send a "lying spirit" into the mouth of some character, who in their own point of view, were not being possessed by a spirit, but were acting in their own interest.
Prophecy has a timeless quality about it, which often combines events from many different periods in order to make one point or describe one prophetic image.
"Day" in prophecy generally refers to a condition or state of being. E.g. "Day of the Lord", "day of disaster", "on that day", etc -- it does not refer to a 24 time period but a condition during which something will happen.
The prophetic passage in question is part of a condemnation of Israel meant to undermine any notion that Israel was blameless before God at any period in time. It does say that God told the people to put away Egyptian gods, but they did not, and God was ready to punish them right in Egypt, but he held back.
The idea of prophecies spoken to the people to put away Egyptian gods is not recorded in Exodus, although there may be some hints about it, in that it was clear that the Plagues were sent against Egyptian gods and that God was making his people separate. That is all that is recorded in Exodus, so this prophecy must refer to either words of Moses or another prophet which were not recorded in the Exodus account or it refers to God speaking directly to the hearts of the people without going through a prophet. We don't know which. But we should not interpret this passage as the giving of the Mosaic Law in Egypt, merely it refers to god condemning idolatry even while the people were in Egypt. Indeed, God condemns idolatry in many different times and places, so it would be odd if the people recieved no notice of this when they were in Egypt.
Here is Hermeneia's commentary:
In comparison with the Pentateuchal narrative it is striking how
decisively Ezekiel summarizes the process of revelation into a single
event. What is there broken up into at least three acts: Yahweh’s
revelation of his name in connection with the call of Moses (Ex 3, 6);
the giving of the promise of the exodus and the settlement in the land
by Moses to the people (Ex 4:30f*; 6:6* in P simply commands this, and
the carrying out is not explicitly narrated) ; the giving of the laws
on the mount of God in the wilderness (Ex 19ff), is here concentrated
at the beginning in the act of revelation to Israel whilst it is still
in Egypt. Also the promise of the land, which according to the
Pentateuch had already been given to the patriarchs, is here included
without any reference back to earlier events. We must be careful here
not to draw conclusions about a different view of the history which
Ezekiel may have received and seek to find a tradition of a giving of
the law to Israel whilst it was already in Egypt. The arbitrary
summarizing is Ezekiel’s own work and is to serve to strengthen his
Zimmerli, W. (1979–). Ezekiel: a commentary on the Book of the Prophet Ezekiel. (F. M. Cross & K. Baltzer, Eds.) (p. 409). Philadelphia: Fortress Press.
In terms of what "that day" refers to, it is the period of time when God decided to bring Israel out of Egypt and he spoke to Moses, so I think it refers to the events in Exodus related to the calling of Moses and God's revelation to him in the desert, not the revelation to Moses on Sinai. Although from the point of prophecy, it can be that these are part of the same revelation, in that we do not know the full extent of what was revelated to Moses when.
The text (LEB):
And you must say to them, ‘Thus says the Lord Yahweh: “On the day of
my choosing Israel I swore to the offspring of the house of Jacob, and
I made myself known to them in the land of Egypt, and I swore to them,
saying, ‘I am Yahweh your God.’
On that day I swore to them to bring
them out from the land of Egypt to the land that I had searched out
for them, flowing with milk and honey—it is the most beautiful of all
of the lands.
Then I said to them, ‘Let each one throw away the
detestable things of his eyes, and you must not make yourselves
unclean with the idols of Egypt! I am Yahweh your God.’
rebelled against me, and they were not willing to listen to me; each
one did not throw away the detestable things of their eyes; and they
did not abandon the idols of Egypt,
and I decided to pour out my rage
on them, to fully vent my anger against them in the midst of the land
“But I acted for the sake of my name to keep it from being
profaned before the eyes of the nations among whom they lived, where I
made known to them before their eyes, to bring them out from the land