Day appropriate

X is a system of sort. The origins of X are believed to be medieval French. It is possible that a clock face was used, with a quarter move of the hand to indicate X. A few changes were made to make X stay within the 60 ticks of the clock face. This is only one of the theories of origin. X also involves usage of Y for representing Z which is also disputed. It is possible that the usage of Y comes from the French expression for the word "the egg". There is also another theory which is the most widely accepted, which i also find quite funny, but i wont tell it now cause it'll give away the answer. Sure some of you out there would have wondered about the peculiarity of X sometime or the other. Give me X,Y,Z.