What it does
Sunny Day switches the weather to harsh sunlight for 5 turns, or 8 if the user holds Heat Rock. The direct effects: Fire-type moves deal 50% more damage, Water-type moves deal 50% less, Solar Beam and Solar Blade skip their charge turn, Thunder's accuracy falls to 50%, and Pokémon can't be frozen.
The ability interactions are why sun teams exist. Chlorophyll doubles the user's base Speed in sun, turning slow Grass-types into speed threats overnight. Flower Gift triggers a form change and boosts every Pokémon on your side's base Attack and Sp. Atk by 50%. Solar Power lifts Sp. Atk by 50% in exchange for 1/8 max HP per turn. On the opponent's side, Dry Skin users lose 1/8 HP per turn in sun — free chip every end-of-turn.
Who runs it
Whimsicott at 32% is the primary Sunny Day user in competitive sun builds. Prankster grants +1 priority to all status moves, so Sunny Day on Whimsicott goes before all priority-0 moves — but Protect (+4), Quick Guard (+3), Fake Out (+3), and ExtremeSpeed (+2) still move first. Unlike ability-based auto-weather, Whimsicott can reset sun after it gets overwritten, which matters mid-game when opponents bring their own weather.
How to use it
The standard open: Whimsicott fires Sunny Day on turn 1 at +1 priority while your Chlorophyll partner acts freely in the sun it just made. With Heat Rock, you get 8 turns — enough to run through most of a doubles match.
Whimsicott's Prankster also gives Tailwind priority, which means it can cover both weather and speed control in one slot. That kind of double-duty support is rare and valuable.
Weather gets overwritten. Bring-your-own rain or sand teams will replace your sun the moment they click their weather move. Whimsicott's advantage is re-setting it at +1 priority — in the same turn the opponent erases your sun, you can usually get it back.
Tips & strategy
The 5-turn clock (8 with Heat Rock) demands turn awareness. If your sun runs out on turn 5 and your Chlorophyll sweeper isn't in position yet, you lose half your team's speed out of nowhere. Track turns out loud if you need to.
The Dry Skin interaction with sun deals 1/8 HP damage to any Dry Skin Pokémon on either side each end-of-turn — yours and theirs. Check your own roster before you commit to a sun build.