Best way to ask for 2 days off that fall just under my probationary 90 day period?

I have just been offered (and accepted!) a new position just two days ago and my first day is on Tuesday. In looking at their employee handbook, it says that there is a 90 day probationary period before PTO can be used.

Despite this, in February of this year -- well before I even applied to this position -- I made plans up for the middle of September. I will need to take off 2 week days total in the middle of the month, specifically this will be 87 days after I start my job on Sep 15 and Sep 18.

My question is: how should I approach this? Should I let my manager know ASAP during my first week and let her make a judgement? Should I contact HR over the weekend? Should I let a week or two go by first?

I'm fully aware I may not be able to take the days off but I'm just curious what my best course of action should be.