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to set one's mind on doing; to get the idea of doing; to make up one's mind
思い立つと止められない。
Once I decide, I can't be stopped.
旅に出ようと思い立って準備を始めた。
I decided to go on a trip and started preparing.
思い立った日が吉日だと言われている。
They say the day you decide is an auspicious day.