They've stated that the Day Pass won't be available on Presto because the 2-hour transfer is viewed as being sufficient for the majority of cases in which a day pass would have previously been useful.
However, I believe Heather MacMillan Brown has said that they plan to have a day pass available on the Presto Tickets dispensed at vending machines and sold at shoppers. Presumably the timeframe would be identical, or almost identical, to the single-ride tickets, as there wouldn't really be a big systemic difference in the infrastructure required; e.g. have separate options on the touchscreen, and encode the "cards" differently.
I can't think of a way for the weekend day pass to work on Presto tickets, because of the faregates being programmed not to allow repeat taps within 5 minutes. Maybe the machines could just dispense 2 paper tickets on weekends for the price of 1, but that runs the risk of people handing out their second ticket to passers-by. Maybe institute a requirement during fare inspections that both tickets must be presented together? But that doesn't take care of subway ridership, and it might not be possible to clearly communicate that to riders. They'll probably just cut it.