SQL Server 2019

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create table testT (Date_c date)
insert into testT values ('2003-03-31')
1 rows affected

insert into testT values ('2014-04-21')
1 rows affected

insert into testT values ('2020-01-13')
1 rows affected

select date_c from testT;


date_c


31/03/2003 00:00:00

21/04/2014 00:00:00

13/01/2020 00:00:00


SELECT Date_c
       , case when  Date_c >= DATEADD(DAY, (9 - DATEPART(dw,CONVERT(date,concat(year(Date_c), '-04-01')))), CONVERT(date,concat(year(Date_c), '-04-01')))
              then DATEADD(DAY, (9 - DATEPART(dw,CONVERT(date,concat(year(Date_c)+1, '-04-01')))), CONVERT(date,concat(year(Date_c)+1, '-04-01'))) 
              else DATEADD(DAY, (9 - DATEPART(dw,CONVERT(date,concat(year(Date_c), '-04-01')))), CONVERT(date,concat(year(Date_c), '-04-01')))
              end as NEXTMONDAY
from testT
where Date_c > '2003-03-31';


Date_c
NEXTMONDAY


21/04/2014 00:00:00
06/04/2015 00:00:00

13/01/2020 00:00:00
06/04/2020 00:00:00

Date_c	NEXTMONDAY
21/04/2014 00:00:00	06/04/2015 00:00:00
13/01/2020 00:00:00	06/04/2020 00:00:00