func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
    let cell = collectionView.dequeueReusableCell(withReuseIdentifier: "MonthCollectionViewCell", for: indexPath) as! MonthCollectionViewCell
    cell.backgroundColor = UIColor.gray
    let firstDate = dates.first
    let index = indexPath.item
    let monthDate = Calendar.current.date(byAdding: .month, value: index, to: firstDate as! Date)
    let monthInt = Calendar.current.component(.month, from: monthDate!)
    let yearInt = Calendar.current.component(.year, from: monthDate!)
    cell.monthLabel.text = String(monthInt)+" "+String(yearInt)
    cell.year = yearInt
    cell.month = monthInt
    let monthDates = dates(self.dates as! [Date], withinMonth: monthInt, withinYear: yearInt)
    cell.colorViews(monthDates:monthDates)
    return cell
}
    func dates(_ dates: [Date], withinMonth month: Int, withinYear year: Int) -> [Date] {
    let calendar = Calendar.current
    let components: Set<Calendar.Component> = [.month,.year]
    let filtered = dates.filter { (date) -> Bool in
        let monthAndYear = calendar.dateComponents(components, from: date)
        return (monthAndYear.month == month && monthAndYear.year == year)
    }
    return filtered
}

class MonthCollectionViewCell: UICollectionViewCell {

    @IBOutlet weak var monthLabel: UILabel!
    @IBOutlet weak var stackView: UIStackView!
    var year: Int?
    var month: Int?

    override func awakeFromNib() {
        super.awakeFromNib()
        for _ in 0...30 {       //for the 31 days in a month...
            let tick = UIView()
            self.stackView?.addArrangedSubview(tick)
        }
    }

    func colorViews(monthDates: [Date]){
        for date in monthDates {
            let dayIndex = Calendar.current.component(.day, from: date)
            let tick = stackView.arrangedSubviews[dayIndex]
            tick.backgroundColor = UIColor.red
        }
    }
}