“pour ainsi dire” is an expression. Litterally, you could translate it as “in order to say so” or “as a matter of speaking”, so “pour” has here the meaning of “for“, but for subtle meaning you should not take it word by word.

“Pour ainsi dire” is used to announce an approximation or an exageration in the use of some terms. In this particular case, I would translate the sentence as (note that I am not sure of the tense to use in English):

I had it almost forgotten

As explained by @radouxju, “pour ainsi dire” is an expression that can, roughly, be translated to the English equivalents so to say or so to speak.

Thus, the phrase

Je l’avais pour ainsi dire oublié…

could be translated as

I had, so to speak, forgotten it (it referring to the fact that “c’est aujourd’hui samedi”.)