All of a sudden is an idiom that is a more poetic way of saying “suddenly.” A common mistake to make, especially for English learners, is to write all the sudden or all of the sudden. On a sudden is a historic but outmoded variant. Currently, all of a sudden is the only accepted usage.
Is it “all of a sudden” or “all of the sudden”?
Although all of the sudden has been used in centuries past, all of a sudden is the phrasing that eventually stuck.
Perhaps it is because Shakespeare used of a sudden in The Taming of the Shrew in 1594, and centuries of grammarians couldn’t help but side with The Bard:
I pray, sir, tell me, is it possible That love should of a sudden take such hold?
Some say that on a sudden is an archaic Scottish variant, but consider that London-born Daniel Defoe used it in Robinson Crusoe in 1719.
Whatever the evolutionary path of this phrase may have been, the only accepted use of it is all of a sudden. You may hear all of the sudden occasionally in informal speech, but don’t let it creep into your writing, since there is no need to attract the disdain of grammar lovers.
The favored horse looked like a sure win until all of a sudden, a dark horse from the back of the pack started gaining.
All of a sudden could be replaced with the adverb suddenly in both of these sentences and they would retain their original meaning.
The favored horse looked like a sure win until suddenly, a dark horse from the back of the pack started gaining.
Why all became part of the phrase is difficult to say. Perhaps it is meant to underscore how completely sudden an occurrence is instead of being just a fraction of completely sudden, and therefore somewhat expected. Idioms are mysterious that way.
Of course, all of the sudden could be called for in a sentence under certain circumstances and be the correct phrase. It just shouldn’t be used to mean “suddenly.”
A small minority of people may use all of the sudden habitually to mean “suddenly,” and you might be tempted to use it if you are used to hearing it. If you revert to it conversationally, no big deal—just be sure to edit it out of your writing.