Is it a matter of convention that 24 September 2017, 17 September 2017, 10 September 2017, 3 September 2017, 1 February 1970, etc. are or were Sundays? Of course, we could have given and can give them a different name. They actually have different names in different languages. We could even have no common name for them. There could be no English language. There could be no Gregorian calendar (at least it could be that no one invented it). And, of course, what people do with Sundays varies greatly from one place or time to another. But it seems to me that it is no convention that these days were, are or will be Sundays. In any case, these thays would always be Sundays.