with t as
 ( select date '2020-02-01' as dt, 0 as flag from dual union all
   select date '2020-02-02' as dt, 1 as flag from dual union all
   select date '2020-02-03' as dt, 0 as flag from dual union all
   select date '2020-02-04' as dt, 1 as flag from dual union all
   select date '2020-02-05' as dt, 0 as flag from dual union all
   select date '2020-02-06' as dt, 1 as flag from dual union all
   select date '2020-02-07' as dt, 0 as flag from dual
 ),tt as (
 select max(case when flag =0 then dt else null end ) start_dt,
        max(case when flag =1 then dt else null end ) end_dt,
        grp_id
 from   (SELECT sum(case when flag =0 then 1 else 0 end) over (order by dt) grp_id,
         t.*
         from t
        )
 group by grp_id
 having max(case when flag =1 then dt else null end ) is not null
 )
 SELECT max(start_dt) FROM tt ;

MAX(START_DT)
-------------
05.02.2020