The concert you have been booked to sing in - and will be paid for - has only one short rehearsal: you know this in advance. You presumably also know that your sight-reading is not even up to managing some really quite simple stuff, and that your knowledge of the repertoire (Christmas carols, FFS) is such that you need your head in your copy to sing the alto part of Hark the Herald Angels Sing. (Which, given it is a boring hymn and the altos basically only sing one note, is saying something.) Do you really think it's professional to accept the booking, and the fee, when you are so clearly out of your depth and not capable of doing the job? We'd have been better off without you, and so would the charity for whom the gig was being put on.