Free XP conversion rate

[DuneDreamer]

I've been recently diagnosed with FXCRS ( the Free XP Conversion Rate Syndrome) by my personal doctor. I know that WG manipulate it from time to time, changing it from 1/25 to 1/35. I believe it was 1/35 a week ago. Does anyone know if this happens on regular intervals, on a particular date or are they deciding it on a secret board meeting behind closed doors and under high levels of security?

[Benser]

It is an event based occurance, they usually announce them in advance, either as part of a major event (For example, WoWS Anniversary) or as part of the monthly events post (An example from the October missions declares the early "weekend discount" from the 5th to the 8th).