No, he definitely means the following day. He continues:
If by next Thursday you still haven't paid, then that 10% turns into 20%. If you leave it for a month, congratulations, your invoice is now 150% of the original.
4 weeks in a month.. so:
day 1: 10%
day 7: 20%
..
day 30: 50%
It only works if he charges 10% the very first day it is late.
That's what each week or part thereof means - you pay for each week and partial weeks are rounded up to whole weeks (so one day = one week, one week and one day = two weeks, one month is four weeks and two days so would be five weeks and so on).
If by next Thursday you still haven't paid, then that 10% turns into 20%. If you leave it for a month, congratulations, your invoice is now 150% of the original.
4 weeks in a month.. so:
day 1: 10% day 7: 20% .. day 30: 50%
It only works if he charges 10% the very first day it is late.