At least in mice, season of birth seems to imprint behavioral cue, affecting the circadian clock (internal time schedule). Some of the effects are notoriously similar to some disorders observed in humans, suggesting that the effect may be extensive to our species (not so distant from mice anyhow).
Cristopher M. Chiarello et al., Perinatal photoperiod imprints the circadian clock. Nature Neuroscience, 2010.
A maybe more easy to comprehend press release is available at Science Daily.
The effects of the environment on the expression of our genes seems to be quite extensive. While we still know too little, there is some steady advance in this most intriguing research field.
While the principal finding is technically limited to the circadian clock its overall effects may be more ample. Co-researcher Douglas McMahon explained that:
We know that the biological clock regulates mood in humans. If an imprinting mechanism similar to the one that we found in mice operates in humans, then it could not only have an effect on a number of behavioral disorders but also have a more general effect on personality.
While it seems easy to compare with simple Astrology, McMahon warns:
It's important to emphasize that, even though this sounds a bit like astrology, it is not: it's seasonal biology!
Well... whatever. What matters is that there is empirical evidence for a quantifiable imprinting effect of psycho-biological effect that is determined by season of birth, and that the extent of this environmental modulation may well extend to all the psyche, affecting the personality.
But if this happens and exactly how is yet to be researched.