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A birthday is the day of your birth, much like graduation day and wedding day. Why is birthday one word?

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    Age. (Of the word, that is.) No really, that's all there is to it. – RegDwigнt Dec 30 '13 at 14:57
  • There is further information surrounding Reg's answer here and here. I've lost the reference I had that stated that compounds usually start off as two- (or three-) word collocations, then often develop into the hyphenated form, and finally into a solid form. The process tends to be quicker in the US. Transitions don't take place at an instant in time, so dual or triple spellings may be acceptable. Some compounds, often for obvious reasons, don't evolve in this way. – Edwin Ashworth Dec 30 '13 at 15:52
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    Unlike "wedding day," a birthday isn't necessarily the day of your birth; it's the anniversary of your birth. – J.R. Dec 30 '13 at 15:53