From the UPS website:
UPS understands the importance of your holiday shipments. All of our drivers are out making deliveries throughout the day and into the evening. The volume of air packages in our system exceeded the capacity of our network as demand was much greater than the forecast, resulting in some shipments being delayed. We expect a vast majority of these packages will be delivered the day after Christmas. We sincerely apologize for any inconvenience. For the most-up-to date information, click here to track your specific shipment’s progress on UPS.com.