Word meaning the opposite of "belated"

Belated has a negative connotation. It is not just "late", so I do not think its opposite is early. "Premature" is a word that denotes early that has a negative connotation.

Early.

Sometimes the obvious answer is the correct answer.

If you say "happy birthday" a day early, it's a "preemptive happy birthday".

@DavidM points out rightly that preemptive implies something is prevented from happening. This could be mean:

• prevention of the shame of forgetting if you wait
• forestalling someone else from being first
• avoiding the recipient's disappointment if you should forget

The OP's use of belated has become very conventional. I suggest part of its charm is its formality. "I want to wish you a preemptive happy birthday," has at least a hint of whimsy.

Edit: after a long and witty repartee, @DavidM and I established that he thinks the adjective preemptive could be misconstrued as to apply to the noun birthday. As a tall brick house is a house that's both tall and made of bricks, not a house made of tall bricks. But I countered that the noun phrase happy birthday is so cohesive that most listeners would think the adjective applied to the pair. He agrees it works spoken, but it still makes him cringe written.

To compete with myself, beforehand is in about the register of belated but is more neutral than premature and might be more fitting since the OP's example was one day early, not 8 months early.

EDIT

But it is not the right part of speech, of course.