Scheduling and managing events using Outlook.com calendar
Welcome to Microsoft Community!
We would like to apologize for not replying in a timely manner. I understand that you are having difficulty in using your Outlook.com Calendar in syncing with your account, let me help you
Please be informed that you need to access your Outlook.com Calendar options by clicking on the gear icon and choose "options" , then choose your "Birthday calendar". Look for the word "Refresh now" and kindly click it. Once you have clicked that option it will sync all of your contacts from your People page.
I hope this will help with your concern and let us know the result.
Thank you,