The riddle
Where Does Today Come Before Yesterday?
Alphabetical order
Dictionaries list words in alphabetical order, and T comes before Y: the 20th letter against the 25th. So "today" sits ahead of "yesterday", and "tomorrow" falls between them: today and tomorrow both start with "to", and "d" comes before "m". Anyone who opens a dictionary finds today, tomorrow and yesterday in that order.
The question sounds like one about calendars, so listeners try to bend the days into an odd order. It is really about spelling.
Ordering by the first letter, then the second, and so on is how every alphabetical list works, from a class roll to the index at the back of a book.
The first English dictionary
Robert Cawdrey's A Table Alphabeticall, printed in London in 1604, is generally regarded as the first dictionary of English words explained in English. It collected hard words and gave their meanings, arranged in alphabetical order for the benefit of readers who were less skilled with language.
Play it forward
Once the answer lands, ask which word a dictionary lists first in a few more pairs. Autumn comes before spring, spring before summer and summer before winter, so the seasons come out in an order that surprises people used to the calendar. Dinner comes before lunch, which no clock would allow.
A timekeeping neighbor
A neighbor riddle asks what always comes but never arrives. It also plays with days, though there the trick is in time itself and not in spelling.