If I'm right, this happens with anybody. I never know what day is it...
But if you really want to know that, try tu put an calendar next to your bed and risk the day before you sleep. This would resolve your problem.
Were you dreaming just before awakening? If so, were you confused about time in your dream?
When we first wake up we all take a few seconds to become fully conscious of our surrounds and the day of the week could be in that class. Some also take a little time to regain physical control of their body... sleep puts us in a state of paralysis. Manly of us, myself include, are not aware of the paralysis, but I understand that is a normal body defense mechanism to prevent physical damage due to a dream inspired physical reaction.