Hastings' Dictionary of the Bible (1898)
WHIT'ED SEPULCHRES. Matt 23:27. It was customary to whitewash the Jewish sepulchres annually, that they might be distinctly seen and avoided, inasmuch as coming in contact with them was the occasion of ceremonial defilement. Num 19:16. This practice gave them a clean and beautiful appearance, and presented a striking contrast to the dark and offensive mass of putrefaction within.