Belated is an adjective meaning made late, having been delayed, sent too late, or long awaited. It applies to something that is late or perceived as late—for example, a birthday card or a delayed apology. It is often used illogically, especially in happy belated birthday and similar phrases. What’s late is the birthday wish, not the birthday, so the adjective is logically misapplied in this phrase.
But the alternatives are usually wordy—for example, I’d like to belatedly wish you a happy birthday is more cumbersome than happy belated birthday—so we should probably consider such phrases involving belated idiomatic and leave it at that. Still, if you want the language of your birthday wishes to be logical, make sure belated applies to the thing that’s late.
Here are a few more examples of the illogical, but more or less conventional, use of belated:
And in honor of his birthday and belated Father’s Day, I’m posting an old post I wrote for him and in honor of all dads out there. [Psych Central]
Juneteenth celebrations: California observes belated Emancipation Day with other states [SCPR]
I want to wish George Washington, the father of our country, a belated happy Father’s Day. [US News and World Report]
And these writers use belated in less questionable ways:
A belated congratulations to the Rebirth Brass Band from me, personally. [OffBeat]
When president Richard Nixon began to withdraw troops in 1969, however, South Vietnam adopted widespread if belated reforms. [Asia Times Online]
Their belated coming-out party gets over on an emotion you don’t often see in hip-hop: equanimity. [Rolling Stone]