Seder
A Haggadah is a book that contains the religious ceremony for the first night of Passover (outside of Israel the ceremony is held on the first 2 nights of Passover).
Twice! The first two nights are Seders, and you read it once at each one.
הגדה means "telling". it is the booklet that is read on Passover during the seder.See the Related Link below.
Both can be used.
Yes, they are spelled the same
The Book of Esther in the Old Testament
Reading and Masculinity
Reading to Ages 3 To 6
New Solution for Reading in the Dark