Well the Snowy Owl appeared at the start of November, so it does look like there are 4 seasons and not 2, or else the Guardling would have appeared at the start of the expansion.
Based on each season being even, that means that spring will start in February, and Summer in May. That is just an educated guess though.
For now the only two seasonal pets are the Owl and Guardling, but I've no doubt that they may add more.
Hope that helps.