If you have linked Google Calendar, the Google Calendar "Birthdays" calendar may be shown.
From the settings screen, tap "Calendar Settings", then tap "Visible Calendars", and uncheck the calendar labeled Birthdays. Birthdays will then no longer be shown on the calendar.
※ Birthday events are "Read Only", so they cannot be deleted individually or recolored in Smart Planner. If a birthday is shown more than once, please fix it in the Google Calendar settings (the birthday information in your contacts).