basically for most people its the begining of the work week, there for the begining of the week in general.
u can use any method really,
people just like to use those days because it is easier to plan your cycle out..
hypothetically if u were to do a 2 week cycle the normal person injecting 500 test/wk would need 4 amps
if u were injecting every 3 days u would need 5 amps,
many of use have alot of things going on in our lives therefore having planned days helps to make one thing less we have to woryy about, well this week injections are on monday and thurs, next week they are on sunday and weds, just gets too confusing...