if you had a sick note given to you on thursday evening that said "i have examned you today and you are not fit for work, and will not be for 1 week" what day would you expect to go back to work?
If you feel better then the following friday. You don’t work fri sat sun mon tues wed thurs which makes the week off and you return to work on the friday
IMO, the sicknote starts when it is given to the sick person. I read it that the OP was the sick person and the doc gave the note to the sick person on thursday evening.. but on re reading, if the OP isn’t the sick person then it depends on the date on the sick note.
so you saw the doc on the thursday and he signed you off for a week and dated it thursday. therefore your first day on the sick cert is friday so seven days counting on from friday puts you to thursday and you go back to work, all being well on friday.