In what order were the Servants summoned for the 5th Holy Grail War?
All of these are explained in the visual novels of Fate/stay night and Fate/hollow ataraxia.


First to be summoned was Berserker, two months before Saber.
Second to be summoned was Caster, one month before Saber.
Rider, Assassin and Lancer are summoned at around the same time, around ten days or so before Saber.
Archer is summoned two days before Saber.


By the way, Saber was summoned during the night between February 2nd and February 3rd.