not the most efficient, but works

;with dates as
(select yr,
case mon when 'jan' then 1 
	when 'feb' then 2
	when 'mar' then 3
	when 'apr' then 4
	when 'may' then 5
	when 'jun' then 6
	when 'jul' then 7
	when 'aug' then 8
	when 'sep' then 9
	when 'oct' then 10
	when 'nov' then 11
	when 'dec' then 12
else 99 end as mon,
case dy when 'sun' then 1
		when 'mon' then 2
		when 'tue' then 3
		when 'wed' then 4
		when 'thu' then 5
		when 'fri' then 6
		when 'sat' then 7
else 99 end as dy,
dyno from @tbl),
table2 as (
select *,
CAST (DATEADD(MONTH,d.mon-1, DATEADD(year, d.yr - 1900,'1900/01/01')) as DATE) as month_start,
DATEPART(weekday,DATEADD(MONTH,d.mon-1, DATEADD(year, d.yr - 1900,'1900/01/01'))) as wd
from dates d
), counted as(
select *, 
case when (SIGN(dy-wd))>0 then (dy-wd)+7*(dyno-1) 
	else (dy-wd)+7*(dyno) end as delta
from table2)

select yr,mon,dy,dyno,
	case 
	when mon =99 or dy =99 or DATEDIFF(mm,DATEADD(day,delta,month_start), month_start)<0
			then 'Invalid date'
	else CAST(DATEADD(day,delta,month_start) as CHAR(10)) end as final 
	from counted
order by final

Tags: tsql beginner challenge