Any large siege had a considerable amount of preparation, and where attempts to break the defences failed, a new strategy would be utilized.

Your work day by day would very much depend on who you are and the skills you possess. From different journals:

"Men who could build and operate machines, mine tunnels, fire weapons for assault parties, and use scaling ladders were the types of men needed to prosecute a successful investment"

"Keeping enemy pinned down by missile fire, and preventing the enemy from getting too close. Engineers or men with specialized skills built and operated sophisticated machinery. Other men acted as guard and formed assault parties."

If you possessed skills in a craft you could be constructing siege weaponry and constructions, or labouring under those craftsmen. Siege machinery was big and heavy, the ladders alone needed several dozen men to move. In order for the craftsmen to do this work, they need to be supplied with the lumber and materials to build with, so if you lacked those skills to create you could be used as a labourer to harvest lumber, and assist in the building processes to prepare for the actual siege.

Containment was one of the biggest jobs for a siege party, in order for them to starve out the defenders they would, "set watches around the castle wall to prevent men and supplies from getting in and out". It's hard to starve people if they have an outside source of food.

Lets not forget everything takes a long time. You might be thrown in to a tunnel to dig if that was part of the general's plan. You could be digging daily for weeks at a time and end up in the situation that happened during the siege of Constantinople where the tunnels were discovered by the defending party and instantly your weeks of effort are undone.

The actual "attacks" wouldn't be the Lord of the Rings style either, where the catapults only start up right as the infantry were about to attack. They would bombard the city for days at a time, that all requires a lot of human effort to keep that working. Even when they get to the walls they can still take a damn while, in the siege of Bruges they fought into the city, broke into the inner keep, and then the defenders holed up inside the church where attackers, "from noon until evening...pounded against the wall" and just getting into the church took 3 weeks.

Now these are all the cool "battle" parts of the siege but a lot of people would be needed to just keep the siege camp running. Just supplying the camp with food is a task that would require manpower as the length of the siege went on. When you have huge armies waiting for the next plan to be put into gear they will mow through a lot of food. In some cases the camped army would actual undertake the job of farming the surrounding region to not lose that seasons crops, to either feed their armies come Fall if the siege had lasted that long, or to not starve the people in their, soon to be, new city.

I am skimming over a lot too, there would be training, equipment production, smiths getting arms ready for battle and all sorts of other miscellaneous tasks that would require your time throughout the day.

You would not just be sitting around.

Sources:

Nicolo Barbaro, Diary of the Siege of Constantinople 1453. Translated by John MelvilleJones. New York, 1969.

Marvin, Laurence W. Common soldiers and the siege of Bruges, 1127. Berry College (1998)

In response to a message from /u/zexez:

The mods are needed here badly.

Hello everyone,

Unfortunately, we have already had to remove a number of poor quality responses in this thread, including many asking about the deleted comments, which merely compound the issue. In this thread, there have been a large number of incorrect, speculative, or otherwise disallowed comments, and as such, they were removed by the mod-team. Please, before you attempt answer the question, keep in mind our rules concerning in-depth and comprehensive responses. Answers that do not meet the standards we ask for will be removed.

Additionally, it is unfair to the OP to further derail this thread with off topic conversation, so if anyone has further questions or concerns, I would ask that they be directed to modmail, or a META thread. Thank you!

If it's not too late I have a follow on. From the opposite end, what an I doing all day if I'm a defender inside the city? Or if I'm a commoner inside the walls?

Follow up Question: (delete if too off topic)

How would a leader of a besieged city decide how to ration food? Do we have sources on how those decisions were made at any sieges? Would the leader have an idea of how long to hope to hold out? (Most sieges wouldn't last past a certain season right? You have to let you troops go home to tend to fields after so long?). Do we know of a formulas for how much to ration out? I'm sure it depended on your stores available at the start.

Another way to phrase the question: If you were a peasant in a newly besieged city, could you expect more rations the first month of the siege than the 6th month or did rationing get pretty severe early (your leaders assume the worse from the beginning)?