How to get 1st and 3rd Saturday and all Sunday between 2 dates using sql - sql

Question

Given a date range, I'd like to return all of the Saturdays and Sundays that fall within that range, with these conditions:
Include Saturdays only if their ordinal position is either the 1st or 3rd Saturday within the month they fall (not within the entire range).
Include all Sundays, along with the ordinal position of that Sunday within the month it falls.
So for example, if the start date is Aug 15, 2021 and the end date is Sep 20, 2021, the output would be:
Dates Saturday Number (in its own month)
---------- ---------------
2021-08-21 3
2021-09-04 1
2021-09-18 3
Dates Sunday Number (in its own month)
---------- ---------------
2021-08-15 3
2021-08-22 4
2021-08-29 5
2021-09-05 1
2021-09-12 2
2021-09-19 3
Then I can take the date range in total (37 days), and subtract the Sundays (6), and the 1st and 3rd Saturdays from each month (3), to end at 28.
Tried this query
DECLARE #sd DATETIME = '2021-08-15' DECLARE #ed DATETIME =
'2021-09-20'
--find first saturday WHILE DATEPART(dw, #sd)<>7 BEGIN SET #sd = DATEADD(dd,1,#sd) END
--get next saturdays ;WITH Saturdays AS (
--initial value SELECT #sd AS MyDate, 1 AS SatNo UNION ALL
--recursive part SELECT DATEADD(dd,7,MyDate) AS MyDate, CASE WHEN SatNo + 1 =6 THEN 1 ELSE SatNo+1 END AS SatNo FROM Saturdays
WHERE DATEADD(dd,7,MyDate)<=#ed
) SELECT * FROM Saturdays WHERE SatNo IN (1,3) OPTION(MAXRECURSION 0)
it does not work properly.
Also Tried this solution
Get number of weekends between two dates in SQL
for calculate week days, but I want only 1st and 3 Saturday and all Sundays

Answer 1

Get a calendar table; it makes this type of business problem a breeze. Here's a simpler one:
CREATE TABLE dbo.Calendar
(
TheDate date PRIMARY KEY,
WeekdayName AS (CONVERT(varchar(8), DATENAME(WEEKDAY, TheDate))),
WeekdayInstanceInMonth tinyint
);
;WITH x(d) AS -- populate with 2020 -> 2029
(
SELECT CONVERT(date, '20200101')
UNION ALL
SELECT DATEADD(DAY, 1, d)
FROM x
WHERE d < '20291231'
)
INSERT dbo.Calendar(TheDate)
SELECT d FROM x
OPTION (MAXRECURSION 0);
;WITH c AS
(
SELECT *, rn = ROW_NUMBER() OVER
(PARTITION BY YEAR(TheDate), MONTH(TheDate), WeekdayName
ORDER BY TheDate)
FROM dbo.Calendar
)
UPDATE c SET WeekdayInstanceInMonth = rn;
Now your query is easy:
DECLARE #start date = '20210815', #end date = '20210920';
SELECT Dates = TheDate,
[Saturday Number] = WeekdayInstanceInMonth
FROM dbo.Calendar
WHERE TheDate >= #start
AND TheDate <= #end
AND WeekdayName = 'Saturday'
AND WeekdayInstanceInMonth IN (1,3);
SELECT Dates = TheDate,
[Sunday Number] = WeekdayInstanceInMonth
FROM dbo.Calendar
WHERE TheDate >= #start
AND TheDate <= #end
AND WeekdayName = 'Sunday';
Results (db<>fiddle example here):
Dates Saturday Number
---------- ---------------
2021-08-21 3
2021-09-04 1
2021-09-18 3
Dates Sunday Number
---------- ---------------
2021-08-15 3
2021-08-22 4
2021-08-29 5
2021-09-05 1
2021-09-12 2
2021-09-19 3
And to get just the number 28:
DECLARE #start date = '20210815', #end date = '20210920';
SELECT DATEDIFF(DAY, #start, #end) + 1
-
(SELECT COUNT(*)
FROM dbo.Calendar
WHERE TheDate >= #start
AND TheDate <= #end
AND WeekdayName = 'Saturday'
AND WeekdayInstanceInMonth IN (1,3))
-
(SELECT COUNT(*)
FROM dbo.Calendar
WHERE TheDate >= #start
AND TheDate <= #end
AND WeekdayName = 'Sunday');

Answer 2

Assuming datefirst is set for Sunday:
(
day(dt) +
datepart(weekday, dateadd(dt, 1 - day(dt))) % 7
) / 7 as SaturdayNumber,
(
day(dt) - 1 +
datepart(weekday, dateadd(dt, 1 - day(dt)))
) / 7 as SundayNumber
For all dates this essentially computes a week number (0-5) within the month, relative to Saturday/Sunday.