Okay, just clearing it up once and for all :-)
Pesahh is a full-length (7 days in Israel, 8 in the Diaspora) holiday.
It can be split up into 3 parts:
"The First Day(s)" (a.k.a. "The Seder Night(s)")
"The Last Day(s)"
Pesahh, in its native Hebrew Calendar, lasts from the 15th through the
21st/22nd of Nisan.
The First and Last Day(s), one each in Israel and two each in the
Diaspora, are full Holidays, with Halakh