Well I can't really tell you what to write (because it's YOUR happiest day, not mine), but I can give you tips on how to get your brain flowing!!!
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Try to Listen to Calm Music! That usually helps you get relaxed!
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Drink some water! Get refreshed!
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Try to remember you happiest memories, ask your guardian/parents/siblings if they remember any happy/fun times!!
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Sorry I couldn't directly help you, but hopefully these help you!
