Yesterday means the 24-hour calender day before the current 24-hour day calender day
It means "Yesterday was Sunday". Remember, the days of the week are NOT capitalized in Spanish.
If tomorrow is Sunday, then today is Saturday. Therefore, yesterday was Friday.
4 days after Sunday is Thursday and this is the day before yesterday. Yesterday would mean you have to advance one day, the day b/f yesterday means you have to advance two days. So the day is Thursday + 2 days or Saturday.
Sunday
Sunday will be in one day
Today is the present. Tomorrow is the future. Yesterday is the past. Tomorrow as yesterday happens the day after. Incidentally, tomorrow never comes.
Sunday Day before yesterday = Wednesday Yesterday = Thursday Today = Friday Tomorrow = Saturday Day after Tomorrow = Sunday
Friday
if on a Monday it would be Sunday
The day must be Thursday because "the day after tomorrow" means two days into the future and two days before Sunday is Friday. that means that yesterday would have been Thursday.
Koinei te Rāhoroi i mua.
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