How do animals/plants change for each season?
There are various parameters that describe change of seasons such as day length, temperature, humidity. But it can be assumed that most of these parameters ultimately depend on one parameter- day length.

The response of plants towards the length of day/night cycle is called photoperiodism (which dictates spring flowering).

This article explains the effect of a gene called CONSTANS(CO) on spring flowering. CO is post transcriptionally regulated by cryptochromes and phyochromes.


  The circadian rhythm in CO mRNA abundance was proposed to represent a photoperiod response rhythm, in which exposure to light coincides with CO expression only under LDs (Long Day) and thereby triggers flowering.
  
  ......
  
  We propose that early in the day phyB promotes the degradation of CO whereas in the evening cryptochromes and phyA antagonize this degradation and stabilize CO protein, 


However, my knowledge of plant molecular biology is not that great and I would advise that you refer the mentioned article and the cross references cited in that.