This is a really basic wireframe of what I'm trying to achieve:
This is what I currently have in Xcode, resulting in 3 cells per row, with no space in between. The date label cell isn't showing up because I don't know how to get cellForItem to recognize it along with the post cells (hence the question):
I have two separate cell classes - one for the date labels and one for the rows of images.
My collection view methods are as follows:
func numberOfSections(in collectionView: UICollectionView) -> Int {
    return 1
}
func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
    return posts.count
}
func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
    let cell = collectionView.dequeueReusableCell(withReuseIdentifier: "postCell", for: indexPath) as! PostCell
    let dateCell = collectionView.dequeueReusableCell(withReuseIdentifier: "dateLabelCell", for: indexPath) as! DateLabelCell
    cell.postImage.loadImageUsingCacheWithUrlString(posts[indexPath.row].pathToImage)
    cell.postID = posts[indexPath.row].postID
    cell.postImage.contentMode = UIViewContentMode.scaleAspectFill
    // Here I'll have to get the date and have the labels display the days of the week properly
    dateCell.dateLabel.text = "Monday"
    return cell
}
But I'm not sure how to manage both cells in the above methods. I'll have to sort the posts by a timestamp so things posted on a certain day get added to the correct day's row, but that's something I'll do another day - for now I'm just wondering how I can get the UI laid out so that the collection view shows a date label, then a row of post cells, and repeat.
Thanks for any advice!

