As I learn about military history I often hear a lot about the campaign season. As I understand it, this refers to a situation where an army of soldier/farmer type chaps go off to wage war in spring, and till the land and harvest the crops the rest of the year.
I first encountered this idea with the early Romans, where it kinda makes sense to me as their first armies were not dedicated fighting forces but made up of the romanticized 'farmer soldier' that the Romans so idolized.
I also heard the term again here when reading about sieges. Somebody mentioned as a consideration for a commander the possibility that the siege would tie forces up for the 'campaign season'.
Imagine I am a general at some point after the early Roman period. What is to stop me from going 'Sod that' and readying my troops and storming all the castles I want in Autumn/Winter while everyone else politely waits for spring? Would my armored warriors not find the enemy soldiers working the fields and have an easy victory? Why/how did it so happen that everyone just kind of fell into this idea of a campaign season, waging war in 'agreed periods' when surprise is so useful in war?
Nothing other than basic practicality. This was not the case of a thin gentleman's agreement where both sides were honorbound to keep from fighting one another but a simple hard fact that came with running a military in the time period. In a time with zero third dimension to warfare you were reliant on supply lines and foraging to keep your army afloat, there were no alternatives, and a supply line meant a long umbellical chord from your home base to wherever you were operating.
As the Autumn and Winter months set in this was not just a case of putting chains on your tires and letting her rip. Rivers flooded, temperatures dropped, food was sparse, and you ran the very real chance of your army dying of starvation and exposure while your enemy sat around in their great halls laughing at you.
Your men can't even reach a large number of areas because of mudslides, snowed in roads, flooded fords, harsh terrain made even harsher, your march has crawled down to a handful of miles a day because there's simply no other way forward except to slug through shitty terrain that the enemy was wise enough not to slug through.
This is to say nothing of the very real morale implication of having your levies and your volunteers growing resentful of you in long, tired, freezing, wet marches eating only what they were able to gather in the Spring, angry because they are not their to protect their families. Even if you managed to get your army to an enemy castle and besiege it, your men are sitting out there in the freezing cold, many of whom are losing fingers and feet by the day, while they listen to the dozen or so men on the inside chanting and singing and drinking while they're warm near a fire. Mutiny was a very real concern in addition to the disintegration of your army.
Then consider the economic impact of your major landcrops simply...not coming to pass that year because you were too busy fighting. You as a general, a lord, a king, thought it would be oh so clever to take a large portion of your fighting age males and walk around in the inhospitable country side with no food, so that when you get back your feels have gone unplowed or understaffed and now your income is severely diminished.
All this for a marginal chance at catching someone by surprise.
If the practicalities can be met and managed, there's absolutely nothing stopping a determined, canny general from exploiting the seasonal element of surprise. In fact, Caesar did just that on several occasions, most significantly in the prelude to the Pharsalus campaign against Pompey in the Civil War, where he caught the opposition by surprise by crossing the Adriatic in winter.
I don't have my sources on hand for this, but will supplement as I can.
I recently encountered an excellent summary of it in Goldsworthy's Caesar. Though the book is in physical form on my shelf at home, by luck, the very page can be found online. The crossing is also covered in Caesar's Civil War, in the first ten sections of Book III, including the fact that the Pompeian forces were in disarray due to the winter, and taken by surprise.