Week Calendar supports an additional birthday alert, standard iOS birthday alert aside.
A) containing text "On the same day at" is Week Calendar's alert which can be found in
Week Calendar Settings > New Events & Templates > Default Birthday Alarm
B) containing text "On day of event" is Apple's alert which can be found in
Device Settings > Calendar (or Mail, Contacts, Calendars in iOS9 and below) >
Default Alert Times (almost at bottom) > Birthdays
If there are two alerts shown, first one is Week Calendar's