create table ts (
	ts timestamp,
	primary key (ts)
);
insert into ts values('2009-09-20 09:10:15');
insert into ts values('2009-09-21 09:10:15');
insert into ts values('2009-09-22 09:10:15');
insert into ts values('2009-09-23 09:10:15');
insert into ts values('2009-09-24 09:10:15');
insert into ts values('2009-09-25 09:10:15');
insert into ts values('2009-09-26 09:10:15');

select date(date_sub(ts, interval mod(dayofweek(ts)+5, 7) day)) from ts;