Ability to run extras is predicated on availability of trains and on the ability to staff those trains (it may bring on dispatching changes which make the schedule weird). On the weekend, availability is not a problem: the majority of the sets available on a weekend are idle on any given weekend day (I do wonder if the ops practice is to try to give most of the sets 1 weekend day on and 1 weekend day off, or if it's more like sets working the whole weekend and then having weekends off). The issue there is staffing, especially if contracts call for 5 days on out of 7 (in which case there's little ability to run extras without at least increasing the chance of cancelling rush hour trains on weekdays).
On a weekday, staffing isn't much of a limitation: it may mean going to the extra board for a day, but the bigger considerations would be managing fueling and also dispatching to keep trains moving. Those are a lot more amenable to something ad hoc: you might have to arrange extra diesel deliveries at the layovers or have some trains the next morning delayed or run odd sets, but the effects are temporary.
From an avoiding chaos standpoint, it's far better to have an event drawing hundreds of thousands downtown on a weekday than a weekend. A lot of the people downtown for that would be downtown anyway on a weekday, and adjusting to go from 60k to 100k is a lot easier than adjusting to go from 15k to 60k.