Are they the same or is there a difference? 
pay day or payment day?
- Thread starter ManOfWords
- Start date
Or, in AE at least, "payday" as one word. I think that's more common than "pay day."
I'd use "payday" in BrE, for the day employees get paid their wages.
In a different context, "payment day" might be possible. The business I work for generally pays invoices just once a week. Although the usual term we use for this is "payment run" - I might ask my colleague what day the next payment run is, for example - I can easily imagine using "payment day" to refer to the date of the next payment run, or I might tell a supplier that our next payment day is Wednesday, for example.
So if I asked you out of the blue ''when is our payment run?'' it would be understood right?!
It would make no sense, since you are not my colleague. However, if someone from within the business who had to deal with suppliers were to ask me, I would understand what they meant immediately. However, I have no idea how widely the term is used, and whether other businesses use different terms. Payments to suppliers is not something I have have ever had any involvement with at other places I have worked.
"Payment day" feels rather ominous. It has a kind of dies irae ring to it, when you have to pay the penalty for all your sins.
It's the day after Judgment Day.
Ah yes, on Judgement Day you only get the invoice.