Refactoring after grouped in Linq
(1). You can create an exntession method to check that item is within the range.

(2). I'd use Count instead of Sum because you just count the number of days in specified interval. It's more natural.

(3). I'd check performance of this query and try to cache result of this join and/or grouping by using some method that requires eager evaluation. ToList() for example. It can help with a lot of traverals of every group in Sum()/Count() methods.

(4). Note for additional information. Even if Sum() in Linq To SQL is a lot faster than Count() you still can have MyCount() implemented as Sum() inside. This will allow you to have a good performance as your senior programmer mentioned and hide this "? 1 : 0" conidition.

 var tempQuery = from prop in propList
                                       join book in booking on prop.PropertyId equals book.PropertyId into j
                                       from book in j.DefaultIfEmpty()
                                       group book by new { prop.PropertyId, prop.PropertyName }.ToList();

 objLeadTimeReportResult = from tempVar in tempQuery
                                 select new LeadTimeReportResult
                                       {
                                           PropertyId = tempVar.Key.PropertyId,
                                           PropertyName = tempVar.Key.PropertyName,
                                           Today = tempQuery.Count(item =&gt; (item.WithinRange()),
                                           Days1_2 = tempQuery.Count(item =&gt; (item.WithinRange(1,2)),
                                           Days3_6 = tempQuery.Count(item =&gt; (item.WithinRange(3,6)),
                                           Days7_10 = tempQuery.Count(item =&gt; (item.WithinRange(7,10)),
                                           Days11_15 = tempQuery.Count(item =&gt; (item.WithinRange(11,15)),
                                           Days16_30 = tempQuery.Count(item =&gt; (item.WithinRange(16,30)),
                                           Days31_45 = tempQuery.Count(item =&gt; (item.WithinRange(31,45)),
                                           Days46_60 = tempQuery.Count(item =&gt; (item.WithinRange(46,60)),
                                           Days60Plus = tempQuery.Count(item =&gt; (item.GreaterThan(60));
}.ToList();