Backtrack to Exodus 12:2 Establishes Abib (now Nisan) as the First month of The Hebrew calendar. It had been established as the month of the first harvest. I don’t know at this point how it was calculated, nor how long this had been in existence. The death angel visited on the 14th and passed over the firstborn when he saw the blood. When were they travelling? The next day and thereon. No manna yet.
The day didn’t have to coordinate with the date.
Jewish tradition records the first manna fell on a Sunday, the 16th of Iyar. That was when their supplies taken from Egypt ran out.