I know that Assassin was summoned after Caster, and in most cases, Saber is the last Servant to be summoned. (The exception is in Heaven's Feel if you count True Assassin's appearance as a separate summoning).
However, I am wondering about the order of summoning of all the Servants, and if possible, the approximate times in which they were summoned.
In the visual novel, the date is shown during cutscenes, and I am quite sure that Saber was summoned on the night of 29th of February 2004, as I remember that the date changed from 29/2 into 1/3 after Shirou returned home with Saber. This can be used to approximate when the Servants were summoned.