Strategically speaking, warring during the winter is a situation that is widely avoided due to major effects on terrain, morale, supply chains, communication and troop physical wellbeing. It's a logistical nightmare.
Unless an armed force is specifically trained and adapted for warfare on these conditions, it's too risky an avenue to pursue. Spring is favourable because of the likelihood of a state of agricultural plenty, better troop morale, more manageable terrain and essentially the opposite of all the factors mitigated by Winter.
An exception is the Russian defeat of Germany on the Eastern Front in WW2. The German forces were completely unprepared for the severity of the Russian winter, and suffered utterly for it.
OP, try to get a copy of Sunzi's The Art of War, preferably with commentary. It doesn't explicitly discuss seasonal warfare, but it does examine all of the factors that are impacted by seasonal change; it should therefore give you a good insight into why these tactical decisions are made.
Absolutely <3 war as a concept, perfect representation of the human condition.