Here are the steps:
Open your project and go to the Main tab. Click Customize Holidays to open the editor.
Here, you will see a list of holiday groups for different countries and their contents. Click New Group and give it a name. Then hit Add and pick either Holiday or Birthday / anniversary. The first type is a regular holiday, the second one is special and will count the person's age automatically. We'll go with the second type.
Give the day a name, set a date and choose the font or background color. If you're creating a birthday, you don't need to enter the age. Just set the date, and the program will count everything automatically. Once done, click OK to save all the changes.
Click OK once again to save the holiday database. Now check the Display holiday names box to see your holidays in the grid. For example, here we added a birthday date:
You can also add a photo of the birthday person right in the holiday database. Learn more about adding pictures into birthday cells in this guide.