Calendar Guts Gist

     // in cell for row
      var day : Date?
        if (indexPath as NSIndexPath).row >= self.getShiftInSection((indexPath as NSIndexPath).section){
            day = self.dateForIndexPath(indexPath)
            style how you want and check if your date contains date with predicate
         }else{
         //style for blank cell
        }
        
        
        // In did select row construct the date like above from the indexPath and check if your objects match date with a predicate
         
         
    func numberOfSections(in collectionView: UICollectionView) -> Int {
        
        return self.numberOfMonthsToLoad(startDate, endDate: endDate)
    }
    func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
        let offset = self.getShiftInSection(section)
        let totalDays = self.daysInSection(section)
        return offset + totalDays
        
    }
          //helpers and calculators
    func dateForIndexPath(_ indexPath : IndexPath)->Date{
        let calendar = Calendar.current
        let date = self.monthInSection((indexPath as NSIndexPath).section)
        var yesComponents = (calendar as NSCalendar).components([NSCalendar.Unit.year, NSCalendar.Unit.month, NSCalendar.Unit.day], from: date)
        yesComponents.day = (indexPath as NSIndexPath).item - self.getShiftInSection((indexPath as NSIndexPath).section) + 1
        let newDate = NSDate(from: yesComponents)
        
        
        return newDate as! Date
    }
      
      
      func getShiftInSection(_ section : Int )->Int{
        
        var offset = 0
        
        if currentSection == nil || section != currentSection{
            let sectionDate : Date = Calendar.current.date(byAdding: .month, value: -section, to: endDate)!
            var oldComp = (calendar as NSCalendar).components([NSCalendar.Unit.year, NSCalendar.Unit.month, NSCalendar.Unit.weekday], from: sectionDate)
            oldComp.day = 1
            let newDay = NSDate(from: oldComp)
            let newComponents = (calendar as NSCalendar).components([NSCalendar.Unit.year, NSCalendar.Unit.month, NSCalendar.Unit.weekday], from: newDay as! Date)
            offset = newComponents.weekday! - 1
            currentSection = section
            self.currentOffset = offset

        }else{
            offset = self.currentOffset
        }
        // adjust offset
        if offset == 7{
            return 0
        }else if offset > 7{
            return offset - 7
        }else{
            return offset
        }
    }

    func getMonthForSection(_ dateIndex : Int)->Int{
        var comp = (calendar as NSCalendar).components([NSCalendar.Unit.year, NSCalendar.Unit.month, NSCalendar.Unit.day], from: endDate)
        comp.month = comp.month! - dateIndex
        return comp.month!
    }
    
    func monthInSection(_ dateIndex: Int)->Date{
        var comp = (calendar as NSCalendar).components([NSCalendar.Unit.year, NSCalendar.Unit.month, NSCalendar.Unit.day], from: endDate)
        comp.month = comp.month! - dateIndex
        return NSDate(from: comp) as Date
    }
    
    func daysInSection(_ section :Int)->Int{
        var  componets = (calendar as NSCalendar).components([NSCalendar.Unit.year, NSCalendar.Unit.month, NSCalendar.Unit.weekday], from: endDate)
        componets.month = componets.month! - section
        return NSDate.numberOfDays(inMonth: componets.month!, forYear: componets.year!)
    }
    
    func getMonthAsDateForSection(_ dateIndex : Int)->Date{
        var comp = (calendar as NSCalendar).components([NSCalendar.Unit.year, NSCalendar.Unit.month, NSCalendar.Unit.day], from: endDate)
        comp.month = comp.month! - dateIndex
        return NSDate(from: comp) as Date
    }
     
     func isToday(_ date : Date)-> Bool{
        if date == self.today{
            return true
        }else{
            return false
        }
    }
    
    func doesDatesContainDate(_ date : Date)->Bool{
        if dates.contains(date){
            return true
        }else{
            return false
        }
    }

    func numberOfMonthsToLoad(_ startDate : Date, endDate : Date)->Int{
        //start and end date
        let time = Calendar.current.dateComponents([.month], from: startDate, to: endDate).month
        if let tm  = time{
            return tm + 1
        }else{
            return 1
        } 
    }

revisit the code as I have added lots of missing bits and fixed some old syntax. it is more up to date but the only difference is that it is for NSCollectionView instead of UICollectionView. Just change the necessary parts and it will work. Good luck. https://gist.github.com/agibson73/8f76d4a9d4381e432a4e94af7f01dd4e