You’re wrong in your assumption that they were using the same calendar that we are.
Lol, no answer given.
Jesus taught us how to count days, did you know that?
According to Jesus:
Today is day one.
Tomorrow is day two
The third day is the day after tomorrow.
Here is my circumcision answer:
A baby born on Wednesday must be circumcised by the following Wednesday, the 8th day.
The third day from Wednesday is Friday and the 5th day is Sunday.
Hopefully you are not caught up in a Sabbath keeping cult like the group you linked to.