A.T. Robertson Commentary Mark 15:42

"And when even was now come, because it was the Preparation, that is, the day before the sabbath," — Mark 15:42 (ASV)

The preparation (παρασκευη). Mark explains the term as meaning "the day before the sabbath" (προσαββατον), that is our Friday, which began at sunset. See discussion on Mt 27:57. The Jews had already taken steps to get the bodies removed (John 19:31).