there is a pattern
If you want an interview in the 2nd half of april, your P3 should reach the consulate by the 15th of Feb. (give and take a couple of days). That has been the pattern so far. Also, for your information, the employment based interviews are mostly scheduled in the second half of the month (there has been some exceptions where they have granted specific requests to schedule an early interview) and the family based interviews are scheduled in the first half of the month. Hope that helps.