From World War I through Vietnam and in use by the Taliban in Afghanistan, the "spring offensive" is a term I've seen used countless times to describe an outburst of attacks which occur this time of year. But why? Does the typical spring climate prove advantageous to combatants? Where did the spring offensive originate, and why is it so globally pervasive?
I don't know what the earliest recorded "spring offensive" is, but this rhythm of warfare was certainly the norm in the Ancient Greek world, as recorded in Thucydides.
I would assume that it's been common since the dawn of large-scale warfare, because the reason is pretty simple: it's hard to attack in the Winter. It's often hard to do anything in the Winter, but generally conditions will favour the defender. The attacker has the problems of moving away from supply and shelter, has to face the elements when travelling and maintaining communications, and so-on. In addition, particularly in a country like Afghanistan, some routes will be impassable in winter due to snows. Look at the 1842 retreat from Kabul for evidence of the challenges of travelling in Afghanistan in the Winter.
(edit: quite apart from the climate, in the North and the South, just the length of the day limits what you can do in Winter.)
So you wait out the winter, hopefully planning and building up strength, before launching an offensive in the Spring.
Why the Spring rather than the Summer or Autumn? Well a commander usually wants as long as possible to press home an offensive. This is especially the case in agricultural societies where the demand for labour peaks with the harvest in early Autumn, or regions with extremes of climate, where Winter is too cold and Summer too hot (again, Afghanistan is a good example).
Of course these rules don't apply to more equatorial countries. In a monsoon climate, for example, it's this season that can make a campaign near impossible. I actually don't know if the Vietnamese spring offensive was determined by the climate or whether it was just coincidence.
''The fifth element: mud.'' - Napoleon
From antiquity to the modern day military logistics have always been hampered by mother nature.
Caesar in Gaul would settle his troops into winter quarters instead of suffering the mud and ice of Russia as the Germans did in 1942.
Inopportune offensives combined with drastic weather conditions is an age-old recipe for military disaster, and almost every commander knew this. That's not to say the rule wasn't violated but when it was usually disaster ensued.
There are some key practical reasons as well - hard ground allowed mobility, and summer/autumn allowed foraging and supplies to be found in the field rather than brought up from reserves.