Final answer:
The labor force in ancient Egypt during the flooding of the Nile was primarily assembled for the construction of pyramids, as this was the time when agricultural work could not be carried out.
Step-by-step explanation:
During historic times, particularly in the late summer and early autumn months corresponding to the annual inundation of the Nile, a large labor force would assemble to work on the construction of pyramids. This was a period when the fields were submerged and agricultural activities were not possible.
The labor demanded for the irrigation system was primarily during the spring, starting in the Coptic month of Țūba, and was focused on repairing damage from the floods. In contrast, pyramid construction work, including transporting massive stones, was most intense around September when the Nile's flooding was at its peak and agricultural work was halted. Notably, the agricultural harvest occurred after the floodwaters receded, and the planting of crops happened after floodwater was returned to the Nile.
Without the cyclic flooding of the Nile, which deposited vital nutrients onto the fields, the irrigation system would not sustain Egypt's agricultural demands, influencing both the economy and the construction feats such as the pyramids.