It is the letter "e". The word day has no "e" in it, the word week has two, the word year has one "e" in it, and the word decade has two in it.
The letter E.
The Letter "e"
e
The letter DDAYDECADECENTURY
The letter "e"
The letter "e."
M
It depends on the decade. Sometimes once, sometimes twice.
The letter E
The letter E
The letter 'E'
The letter DDAYDECADECENTURY
Letter e