Try this:
var minDateInclusive = new DateTime(2018, 6, 1);
var maxDateExclusive = new DateTime(2019, 1, 1);
var query = from ph in _webshopDbContext.FXXLPurchaseHeaders
join pl in _webshopDbContext.FXXLPurchaseLines on ph.No_ equals pl.Document_No_
where pl.Brand == "CUBE"
&& ph.Ship_to_Name_2 == "Zentrallager"
&& ph.Order_Date >= minDateInclusive
&& ph.Order_Date < maxDateExclusive
orderby pl.Size, pl.Colour
select new LoadResults ...