Hi How would you say, "Have a great day, everyone"?

Bíodh lá iontach agaibh, gach duine!

Wait for the more learned to come along....

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Sounds a bit strange to me to say "agaibh, gach duine" since "agaibh" is plural and "gach duine" is singular (each person).
I'd simply say "Bíodh lá iontach ag gach duine" (or in Ulster Irish "Bíodh lá iontach ag achan nduine")
or: Bíodh lá iontach agaibh uilig

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Is fearr Gaeilg na Gaeltaċta ná Gaeilg ar biṫ eile
Agus is í Gaeilg Ġaoṫ Doḃair is binne

Fair point. How about 'Bíodh lá iontach agaibh go léir!'?

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Looking around I've found -
(Go raibh) lá breá agat/agaibh
Bain taithneamh as do lá

I presume the second would have to be: Bainigí taitneamh as bhur lá as the request was to more than one.

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Go raibh maith agaibh Tá sé le haighaidh Facebook; bá feidir liom "jwefhgefveu" a scríobh agus ní bá a fhios acu an difríocht, but I figured I'd ask anyway! (I know that was probably really bad, cause I was half guessing XD)

Yes if it were in the plural of course it would be that.
Perhaps somebody more knowledgeable could tell us whether this idea of - have a good/great day - would be said in Irish at all or would there be an expression such as - God's blessings for this day to you ?

That is a very good point