Could you please tell me if there is a difference between Sunset to sunset, and sundown to sundown?
Is there a difference between Sunset to sunset or, sundown, to sundown? Biblically, no, because those words/phrases are not in the Hebrew scriptures. "Sunset to sunset" and "Sundown to sundown" are only English approximations to the Hebrew.
Various Bible translations are not even consistent as to which English idiom is used in the translation. In common usage however, i.e., in everyday life, most people understand the two phrases as synonyms.
But some pockets in some cultures use the terms slightly differently, for example, "sunset to sunset" is from the moment the sun has set to the moment the sun has set the next day, while "sundown to sundown" is taken to mean "the sun has set and it's now getting dark".
The implication is that if something needed to be done while it is still light, "sundown" means it's too late, for the sun has set and darkness is setting in. Back to the scriptures, some people do try to argue that there is a difference between "sunset" and "sundown", but the Hebrews did not think that way.
To a Hebrew, the late afternoon is "erev", usually translated into English as "evening". But the Hebrew "erev" means "mixture", which makes sense because in the late afternoon and early evening after sunset, the sky is a mixture of light and dark. So to a Hebrew, the time after sunset when the sky is getting dark is also "erev."
In English that is "evening" so that is what is usually used in translations. Example: In Exodus 12:38 we read "A mixed [erev] crowd also went up with them, as well as livestock in large numbers, both flocks and herds" (CJB). In Genesis 1:8 we read "So there was evening [erev], and there was morning, a second day" (CJB). So we see that the chosen meaning of "erev" into English depends on the context and use, while the English still does not capture the actual Hebrew meaning!
The use of sundown and sunset have similar problems. Hebrew does not say "sunset" or "sundown"! Period. Example, in Exodus 17:12 we read "the one on the one side and the other on the other; so that his [Moshe's] hands stayed steady until sunset." (CJB). But for sunset, the Hebrew uses "shamesh", which means "brilliant" and therefore refers to the sun. The word's use and context in the verse means "until the going down of the sun".
In English, "until sunset" is pretty close, but it is NOT what the Hebrew says! The Hebrew permits that Moshe's hands became heavy as the sun was descending, not that the event was "at sunset". In Exodus 22:26 we read "If you take your neighbor's coat as collateral, you are to restore it to him by sundown," (CJB).
Again, here the Hebrew is "shamesh"! So once again the English is close, but does not convey the intent of the Hebrew which is that the cloak is to be restored as the work-day is ending and the sun is going down.
One more example: In Leviticus 23:5 we read: "In the first month, on the fourteenth day of the month, between sundown and complete darkness, comes Pesach for Adonai." (CJB). But here, for "sundown" in the Hebrew is again "erev"! So the actual meaning is "between the evenings", and NOT "between sundown and complete darkness".
So, we can understand the English "sunset to sunset" and "sundown to sundown" to mean the same thing. The sun is setting or has set and the day has changed. Our activity, especially if the day is Friday, for example, should begin to wane as the sun is descending in the West, and we are not to resume our activity until a full day later, and that would logically mean that the sun has actually set on Saturday, and the old day is over and the new day has begun.