@anyatama, @Dhammanando, and @sujato.
I’ve been meaning to get back to this, but as usual it takes time!
First, I am quite satisfied with translating ratta as “day”. The idiom is the same in Pali as it is in English, just that a different word is used. I do not wish to add too much interpretation to the translated text.
But the interpretation is certainly interesting. I remember thinking how silly it all seemed when I first heard of monks taking their saṅghāṭis along when they went out to urinate around dawn. It seemed like a typical example of keeping the letter of a rule, but without questioning whether their actions were sensible. This led me to investigate whether this rule perhaps should be interpreted differently.
I then realised that the Vibhaṅga does not actually say you have to be with you three robes at dawn, but that the offence occurs at dawn. This is quite different. The offence occurring at dawn is a standard description in the vinaya to show when the offence occurs given that other conditions are fulfilled. For example, one commits a nissaggiya pācittiya under bhikkhu NP1 at dawn on the tenth day. The important point is not what you do at dawn, but what you have done in the previous ten days. And there are a number of similar instances in other rules. Based on this, it seems likely that the stipulation of dawn in NP2 does not relate to what you do at dawn.
Indeed, the rule itself says that you commit an offence if you are separated from your robe for one night/day. On the face of it, it is a bit curious to interpret this to mean dawn. Rather, if we assume the normal meaning of ratta as a period 24 hours, then the offence is incurred at dawn only if you have been away from your robe for the preceding 24-hour period. To me this gives a far more satisfactory interpretation of the rule, which also happens to agree with the meaning of dawn in the other rules. In practice, you would then have to check on your three robes at least once in every 24-hour period, which often might mean staying with your robes at night. In this way we avoid the standard rather artificial and often inconvenient interpretation.