Well... It's a some-what long process involving the Underground. First, you need to have at least ten Red Shards. These can be dug up in the Underground. Go to Route 212, and there will be a house there. A lady will give you a TM for every 10 shards of the same color. Sunny Day for Red, Rain Dance for Blue, Hail for Green, and Sandstorm for Yellow.
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use a TM
By using Sunny day, Hail, Rain Dance. Moves that It can learn
TM11 Sunny Day can be found in the Safari Zone.
Well, it's pretty hard but try migrating Pokemon that has a move Sunny Day from Pokemon Fire red, Leaf green, Ruby, Sapphire and Emerald. Or just trade with a friend with that move. Forgive this answer. Though I've played this game 4 times with different Pokemon I forget where to find it.
Once a a day in Pokemon Platinum