Rather than counting days of the month individually, early Romans had 3 principle days in a month & used to measure days from there. These were:
Kalends – beginning of the month
Nones – roughly one week into the month
“X” – middle of the month
While the first two are obscure, the third “X” is widely popular in a particular context which is regarded as a day of tragedy.
Identify X & why it might be popular?