While marking final exams my mind flew off on this tangent: time (as I was procrastinating, this seemed a good tangent).
When does a day begin? For practical purposes, when you get up in the morning and it ends when you go to bed at night. For scientific purposes - that is, in the never ending effort to carve all existence up into verifiable, countable, examinable, testable, categories, the day begins at one minute after midnight and ends at midnight 24 hours later. For religions, there are similar idiosyncrasies. If you are Jewish, for example, the day begins at dusk and runs to dusk 24 hours later. So Saturday night is really Sunday and Sunday night is Monday. For Christians the day used to follow the 'get up in the morning, go to bed at night rule, but a few decades back in order to either attempt to smooth out some of the inconsistencies between the Old and New Testaments, or to give recognition to the Jewish roots of Christianity, the dusk to dusk rule was followed. Sort of. Not really. Almost. If you are Catholic you can go to Mass at 5 on a Saturday evening and satisfy the rule for attending Sunday Mass. Get home after six and you are free to party, drink, watch a movie, stay up late, and then sleep late on Sunday all having fulfilled this obligation. But. You may also attend a 7 p.m. Mass on Sunday evening, which if this new ordering of time means anything, should not be Sunday Mass but Monday.
Oh well. O Tempus, O Mores! (Cicero, 1st Oration against Catiline: a piece I had to sweat over in Latin class at university)