There's a superstition that turning a mattress should not be done on a Friday
(Or some other day; point of superstitions is I guess to sharpen the memory,
not make people neurotic, as sometimes happens).
But how often do people turn their mattresses?
I think it is good to turn them upside down, backside front, and spray them
with something like Zoflora, which freshens and also kills bacteria.
About once every two weeks seems right.
You will sleep more comfortably, and the bed will last longer.