Lasting the entire day; continuing throughout daylight hours.
Compound of 'day' + 'long,' following the pattern of other duration words like 'lifelong' and 'yearlong.'
The human preference for 'daylong' over 'day-long' or 'day long' shows how English creates compound adjectives so often that we've developed an intuitive rule about when to squash words together—something you learn without ever being taught.
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