Typically with major streams such as a Nintendo Direct the Mario makers do give fans advance warning of when the upcoming broadcast will be taking place.
However, they do also have form in suddenly dropping a broadcast out of the blue without any prior announcement.
Nintendo did this with the Direct Mini in March this year, and they also did it with the Direct Mini that aired in January 2018.
It would be surprising if Nintendo did this with a proper Nintendo Direct, as the next broadcast is rumoured to be, but never say never.
In terms of possible timings for the July 2020 Direct, the Direct Mini in March went live out of the blue at 2pm UK time.
So that could be a possible start time for the upcoming, rumoured Nintendo Direct.
source: express.co.uk