with dates as (
  select to_date('2020-09-12', 'yyyy-mm-dd') as dat union
  select to_date('2020-09-13', 'yyyy-mm-dd') as dat union
  select to_date('2020-09-18', 'yyyy-mm-dd') as dat union
  select to_date('2020-09-19', 'yyyy-mm-dd') as dat union
  select to_date('2020-09-20', 'yyyy-mm-dd') as dat union
  select to_date('2020-09-26', 'yyyy-mm-dd') as dat union
  select to_date('2020-09-27', 'yyyy-mm-dd') as dat)
--
--
select dat
     , min(dat) over(partition by grp_id) as date_begin
     , row_number() over(partition by grp_id order by dat) as rn
  from (
         select dat
              , dat - row_number() over(order by dat)::int as grp_id
           from dates
       ) v
 order by dat