"index out of range" error. Weekday string should be as follows.

[JoelGodin]

When going from Thursday to Friday, program is trying to access in w the eight element. List is only seven elements long.
I replaced :
class RTC:
w = ["SAT","SUN","MON","TUE","WED","THU","FRI"]

with:
class RTC:
w = ["null","SAT","SUN","MON","TUE","WED","THU","FRI"]

Or weekday = self.w[self.bcd2bin(buffer[3])] should be
weekday = self.w[(self.bcd2bin(buffer[3]) - 1)]

[JoelGodin]

Used 2nd solution and requested Pull.