I would say that is not necessarily Christian in general, but more specifically RCC.
It is also a holiday in the Anglican Communion and among the Eastern Orthodox (though on a different date as their calendar has shifted), so about 65% of the world's Christians celebrate All Saints' Day. Not observing All Saints is a peculiarity specific to the minority of Christians known as Reformed or Protestant Christians.
This is not true. Acension Day is celebrated by most of Christedom including Roman Catholics, Lutherans, Orthodox, Epsicopalians, Anglicans, and even some Methodists who bother to follow the liturgical calendar.