Sorry to ask again...and sorry if it's a stupid question...but I don't know the answer...so I'll appreciate your wisdom...

My flight is on Saturday 19th... The window period of 4 days to call for an upgrade...does it begins exactly on tuesday morning (tue-wed-thur-fri then you have your 4 days...) or is it on wednesday (wed-thu-fri-sat)???

Thanks...and sorry for this "easy" question...