SET DATEFIRST 1
DECLARE @last_month int = 6;
DECLARE @last_year int = 2018;
DECLARE @FirstYear int = 2015;
Declare @Last_Day datetime = CONVERT(datetime, CONVERT(varchar, @last_month) + '/1/' + CONVERT(varchar, @last_year), 101);
Declare @Last_Week int = datepart(isowk, dateadd(month,1,dateadd(day,1-day(@Last_Day),@Last_Day))-1)-1;
Declare @date datetime = CONVERT(varchar,   '01/01/' + CONVERT(varchar, CASE WHEN @last_year != DATEPART(year, getdate()-10) THEN (DATEPART(year, getdate()-10)) ELSE @last_year END), 101);
Declare @sDay int = DatePart(dw, @date);
Set @date = @date - @sDay + 1 + CASE WHEN @sDay > 4 then 7 else 0 END;

SELECT ISNULL([S1].[RetailID],[S2].[RetailID]) AS [RetailID], ISNULL([S1].[DistributorID], [S2].[DistributorID]) AS [DistributorID], ISNULL([S1].[Date], [S2].[Date]) AS [LastDate]
FROM (
	SELECT [T1].[RetailID], [SS].[DistributorID], [T1].[Date]
	FROM (	SELECT MAX([S].[ID]) AS [ID], [S].[RetailID], DATEADD(dd, [S].[Week] * 7 - 1,@date) - 2 AS [Date]
			FROM (SELECT [year], MAX([Week]) AS [Week], [RetailID]
				FROM [tblSaleWeek]
				WHERE [Packs_sold] > 0 AND ([year] = @last_year)
				GROUP BY [RetailID], [year]
				) AS [SW]
				INNER JOIN [tblSaleWeek] AS [S] ON [SW].[year] = [S].[year] AND [SW].[Week] = [S].[Week] AND [SW].[RetailID] = [S].[RetailID]
			GROUP BY [S].[RetailID], [S].[Week]
		) AS [T1]
		INNER JOIN [tblSaleWeek] AS [SS] ON [SS].[ID] = [T1].[ID]
	) AS [S1]
	FULL JOIN (
	SELECT [T2].[RetailID], [SS].[DistributorID],  [T2].[Date]
	FROM (	SELECT MAX([S].[ID]) AS [ID], [S].[RetailID], DATEADD(mm, 1, CONVERT(datetime, CONVERT(varchar, [Month]) + '/1/' + CONVERT(varchar, [year]), 101)) - 1 AS [Date]
			FROM (SELECT [RetailID], MAX(CONVERT(datetime, CONVERT(varchar, [Month]) + '/1/' + CONVERT(varchar, [year]), 101)) AS [Date]
				FROM [tblSale] 
				WHERE [Packs_sold] > 0 AND [year] BETWEEN @FirstYear AND @last_year - 1
				GROUP BY [RetailID]
				) AS [SM]
				INNER JOIN [tblSale] AS [S] ON [SM].[RetailID] = [S].[RetailID] AND YEAR([Date]) = [S].[year] AND MONTH([Date]) = [S].[Month]
			GROUP BY [S].[RetailID], [S].[year], [S].[Month]
		) AS [T2]
		INNER JOIN [tblSale] AS [SS] ON [SS].[ID] = [T2].[ID]
	) AS [S2] ON [S2].[RetailID] = [S1].[RetailID]
order by 3 desc