via #iosdev on freenode.net: User wanted the first and last column of a 7-Column collection view to be wider than the middle columns

UICollectionView Cell Sizing

// Intent: Create a 7-Column collection view where the first and last column are wider than the middle columns
// Solution: Via the UICollectionViewDeleateFlowLayout calculate the correct size of the cells

const NSUInteger daysInWeek = 7;
const CGFloat cellHeight = 54.f;
const CGFloat sizeRatioForWeekStartEnd = 0.20;
const CGFloat sizeRatioForMiddleDays = (1 - (2 * sizeRatioForWeekStartEnd)) / (daysInWeek - 2);

- (CGSize)collectionView:(UICollectionView *)collectionView layout:(UICollectionViewLayout *)collectionViewLayout sizeForItemAtIndexPath:(NSIndexPath *)indexPath {
    CGSize cellSize = CGSizeZero;
    
    NSUInteger startOfWeek = 0;
    NSUInteger endOfWeek = 6;
    NSInteger currentDay = indexPath.row % daysInWeek;
    
    NSUInteger spacerColumns = daysInWeek - 1;
    CGFloat totalWidthOfSpacerColumns;
    
    UICollectionViewFlowLayout *flowLayout;
    if ([collectionViewLayout isKindOfClass:[UICollectionViewFlowLayout class]]) {
        flowLayout = (UICollectionViewFlowLayout*)collectionViewLayout;
        totalWidthOfSpacerColumns = spacerColumns * flowLayout.minimumInteritemSpacing;
    } else {
#warning Handle this error gracefully in production
        NSLog(@"collectionViewLayout should be a UICollectionViewFlowLayout");
        abort();
    }
    
    CGFloat spaceAvailableForDayCellsInRow = CGRectGetWidth(collectionView.bounds) - totalWidthOfSpacerColumns;
    
    CGFloat cellWidth;
    if (currentDay == startOfWeek || currentDay == endOfWeek) {
        cellWidth = spaceAvailableForDayCellsInRow * sizeRatioForWeekStartEnd;
    } else {
        cellWidth = spaceAvailableForDayCellsInRow * sizeRatioForMiddleDays;
    }
    cellSize.width = cellWidth;
    cellSize.height = cellHeight;
    
    return cellSize;
}

Hey, it worked now, started from scratch! Thanks! And the reason why it was messed up sometimes was of screen sizes, sizeForRationMiddleDays en sizeForRatioWeekStartEnd might be floats, which it ends up rounding, but that makes it possible that there was 2px of free space that messed up the entire layout.