All weekend long, starting today, we can get 5 rewards per day from battling the NPC team leaders (instead of the usual 1), and we can get another 5 rewards per day from battling real fellow trainers (instead of the usual 3). The guaranteed Sinnoh Stones are tomorrow (Saturday) only (in parts of the world where this Community Day is Saturday), but even they will be throughout the day, midnight to midnight, not only during the 3-hour Community Day window.
Also, if you haven't heard, stat buffs are coming! Suddenly, Ancient Power on a Mamoswine looks much, much sweeter than it did a few days ago. I have an appointment that's going to make me miss half of Community Day, and, despite wanting a shiny watermelon Swinub of my own -- and all the boosted stardust! -- I was very blase about missing out ... until the stat buffs were announced. Poor timing for me! Ah, well. I do have several saved-up Swinub from the ice event to evolve (without events, ice types are vanishingly rare here).