Why it happens on this ground specifically
Clay is the reason mud season matters here. It holds water instead of letting it drain through, so ground that would firm back up quickly somewhere sandier stays soft for weeks after the thaw starts. Layer that onto a stretch of ground that spent the winter freezing and thawing, and the top foot or two can turn to soup even while the ground underneath is still mostly solid.
Elevation plays a part too. Ground that sits higher and stays shaded longer through the day holds onto frost and moisture after lower, sunnier ground has already dried out, so two sites a few miles apart can be on completely different timelines.
What it does before a truck ever shows up
Access is the first thing mud season touches. A driveway, a yard, or a gravel farm road that carries a truck fine in July can rut and bog down a loaded mixer or our trailer pump in soft ground. That is a site problem, not a concrete problem, and it is why we walk a site ahead of a pour scheduled during that stretch rather than assuming the same access will hold up.
What it does to base prep
Prep is where mud season shows up hardest. Crusher run stone spread over soft, saturated clay subgrade does not compact the same way it does over firm ground, and a base packed on top of mud is not doing the job a base is supposed to do. Getting real compaction on a site that is fighting the thaw can mean extra stone, extra time letting the ground firm up before the base goes down, or working around the softest parts of the site rather than driving straight through them.
What it does to the pour itself
Once a good, compacted base is down, the pour is not really different because of mud season. The concrete does not know what the ground looked like a week ago. What can move is the date, not because concrete cannot go down in early spring, but because a truck or a pump cannot always get to where it needs to be until the ground firms up enough to carry the weight.
The one exception is a job that needs to move a lot of material across soft ground before the pour, a large addition or a job with a long haul from the road. That kind of access problem can shift a date even after the pour spot itself is ready, simply because getting stone and steel to the site is its own challenge on soft ground.
When it lets up
There is no fixed date for it. It depends on how wet the winter was, how fast the thaw comes, and how much sun a particular site gets to dry out. We read the ground on the day, the same way we read the forecast, rather than working off a calendar. If your site is fighting mud when you want to schedule, tell us and we will walk it and talk through timing rather than guessing over the phone.