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.