Physicists use the wave theory on Mondays, Wednesdays and Fridays and the particle theory on Tuesdays, Thursdays and Saturdays
William Henry Bragg; quoted in Dictionary of Scientific Quotations by Alan L. Mackay, Institute of Physics Publishing, Bristol, 1994, p. 37 
Variant: On Mondays, Wednesdays and Fridays we teach the wave theory and on Tuesday, Thursdays and Saturdays the corpuscular theory.
Quoted in Physically Speaking: A Dictionary of Quotations on Physics and Astronomy by C.C. Gaither, 1997, ISBN 0750304707. 
unsourced variant: God runs electromagnetics by wave theory on Monday, Wednesday, and Friday, and the Devil runs them by quantum theory on Tuesday, Thursday, and Saturday.