It's interesting that all of the days of the week except for Thursday are based on Friday.
Well, juma also means "week" even though its meaning is "Friday" in Arabic. The week is seen to begin on Saturday in Swahili, so it's week-1, week-2 etc.
Yes, but the number indicates the number of days after Friday (jumatatu for Monday, for example).
Alhamisi is 'the fifth day' in arabic - hamsa..