Two children, who were all tangled up in their reckoning of the days of the week, paused on their way to school to straighten matters out.
“When the day after tomorrow is yesterday,” said Priscilla, “then ‘today’ will be as far from Sunday as that day was which was ‘today’ when the day before yesterday was tomorrow!”
On which day of the week did they have this conversation?
Sunday.
When the day after tomorrow is yesterday, today will be three days hence, just as when the day before yesterday was tomorrow carries us back three days from now, which must be Sunday, to be midway between the two “todays”.
Alternatively, we can rephrase the riddle to:
in 3 days
we will be as far from Sunday
as we were from Sunday
3 days ago
Sunday is the only day that satisfies that.