+1:thumbsup:The 28 days is just an old wife's tale. Concrete actually cures for years. It cures the fastest in the first couple of days and slower later on. The 28 days only refers to one of the time dates (7, 14, 28 and 56 days) for cylinders to check the concrete mix under lab curing conditions.
Depending on the load, loads can be placed on it almost immediately.
I frequently see the first course or two of block for a basement laid the afternoon of the pour. This is done to make sure any rain does not swamp the basement footings and depositing dirt/mud that can take a long time (and money) to clean when the discovered the next day. Anything in an excavation can get very messy and unworkable and delay anything.
A typical basement footing or trench type footing has very little load on it until the structure is up and backfilled. If it is a trench type footing with not unbalanced soil, backfilling improves the curing conditions.