you run the risk of killing off your bio media in your filter without flow.
i have no idea of the time it takes to do this though.
plus i would think in a smaller tank without flow, there would be less oxygen exchange with the surface. that could be a risk also.
If you only have plants in your tank, a "pond day" probably wouldn't hurt anything.
With fish, your filter should run 24/7/365.
Ponds without filters, are large volumes of water, with large surface area, and most fishes kept in ponds are hardier, like goldfish, that can normally withstand a less clean environment and lesser water quality than most other tropicals.