Schema Theory

Sales LY = 
CALCULATE(
    SUM(Sales[SalesAmount]),
    DATEADD('Date'[Date], -364, DAY)
)

Change vs Last Year (%) = 
VAR CurrSales = SUM(Sales[SalesAmount])
RETURN
IF(
    ISBLANK(Sales[Sales LY]) || Sales[Sales LY] = 0,
    BLANK(),
    DIVIDE(CurrSales - Sales[Sales LY], Sales[Sales LY])
)

-- Daily comparison joined on same weekday last year
SELECT
  curr.OrderDate AS Date,
  curr.TotalSales AS SalesThisYear,
  prev.TotalSales AS SalesLY,
  CASE 
    WHEN prev.TotalSales = 0 OR prev.TotalSales IS NULL THEN NULL
    ELSE (curr.TotalSales - prev.TotalSales) / prev.TotalSales
  END AS ChangeVsLY
FROM
  SalesSummary curr
LEFT JOIN SalesSummary prev
  ON prev.OrderDate = DATEADD(DAY, -364, curr.OrderDate);

-- Using window function:
SELECT
  OrderDate,
  TotalSales,
  LAG(TotalSales, 364) OVER (ORDER BY OrderDate) AS SalesLY,
  CASE 
    WHEN LAG(TotalSales, 364) OVER (ORDER BY OrderDate) = 0 
         OR LAG(TotalSales, 364) OVER (ORDER BY OrderDate) IS NULL THEN NULL
    ELSE (TotalSales - LAG(TotalSales, 364) OVER (ORDER BY OrderDate)) 
         / LAG(TotalSales, 364) OVER (ORDER BY OrderDate)
  END AS ChangeVsLY
FROM SalesSummary;