I remember when I was in the civil service back in the 90's, over a 16 month period we were not paid on time on 3 occasions, one being when the senior manager signed off a paydate of 31/02/93. So sometimes these sort of things happen, my old man when he worked for Manchester council had his pay stuck at a closed bank, the payment details were posted off to the branch but it had been closed for two weeks he had informed the council HR department of the change of address along with the 50 or so others who had accounts at that branch all to no avail.
Perhaps the club have changed the pay date to end of the month and the player has not realised.
As BC says not paying on time is a Breach of Contract under employment law, but this will soon not be the case when the new employment laws will mean that an employer can pay you when he wants and if he wants to regardless of your contract.