Timing of stutter/blocking and fluency

Timing of stutter/blocking and fluency My son has a stutter which is mostly blocking and some physical tics. The physical tics tend to precede his periods of dysfluency. Now his periods of fluency and dysfluency are very predictable. He will begin to stutter again in mid-December and become more fluent (I don't remember when) and begin to stutter again mid-June and then become almost completely fluent in October until mid-December. It has been this way for the past 3 years. Does anyone else do this or have any idea why it is so rhythmic. His birthday is in June and Christmas is in December but I can't think of a single reason why he would be anxious during those times. We are a highly functional, calm, supportive, family.