What does having an R in the month mean?

Question:

What does having an R in the month mean?

Both times I've heard it near the topic of oysters.

Best Answer - Chosen by Asker:

Months without an R are the summer months. It has long been said to avoid oysters during these warmer months. There were two reasons: spawning and safety.

Oysters from the summer months were generally smaller and of lesser quality vs oysters in cooler waters. (Spawning)

Warmer waters encourage bacteria. (Safety)

This is not a total myth.