Optimize query which took hours to be executed

How could optimize the following query :
    
    
     SELECT 
                CASE WHEN COUNT(distinct t1.[datecol]) = 0 
                        THEN 0 
                        ELSE SUM(convert(float, Quantité)) / count(distinct t1.[datecol]) 
                END as QTEMOY,
                CASE WHEN COUNT(distinct t1.[datecol]) = 0 
                        THEN 0 
                        ELSE SUM(convert(float, [Prix de vente TTC])) / count(distinct t1.[datecol]) 
                END AS CAMOY,
                t1.[Code Article],
                t1.[Code Structure],
                t1.[Code Site]
    from [Vente ] t1
    outer apply
    (
      select distinct top 28  t2.[datecol]
      from [Vente ] t2
      where t2.[Code Article] = t1.[Code Article]
        and t2.[Code Structure]=t1.[Code Structure]
        and t2.[Code Site]=t1.[Code Site]
        and not exists   (SELECT   [Code Article]
     FROM     [Promotion ]  
    
     where t2.datecol between 
            [Date Debut Promo] and  [Date Fin Promo]and  t2.[Code Article] = [Code Article] )
      order by t2.[datecol] desc
    ) t2
    where not exists   (SELECT  t1.[Code Article]
     FROM     [Promotion ]  
    
     where t1.datecol between 
            [Date Debut Promo] and  [Date Fin Promo]and  t1.[Code Article] = [Code Article] )
      and (t1.[Code Article] is not null)
      and (t1.[Code Structure] is not null) 
      and (t1.[Prix de Revient] is not null ) 
      and t1.datecol = t2.datecol
      and t1.[Code Article] in( select [Code Article] from [Reference] t )
      group by  t1.[Code Article],
                t1.[Code Structure],
                t1.[Code Site]

The query took hours executed HOW TO OPTIMIZE it?