Words:思い立ったOmoitatta = something comes into one's mind; make a decision; the adjective form of the verb Omoitatsu /
がga = nominative joshi particle /
吉日Kichi-jitsu = a lucky day
English Equivalent:
(There is) no time like the present. / Tomorrow never comes. / Never put off until tomorrow what you can do today.
Notes:Kichi-jitsu is a lucky day ancient people used to select based on divination. The phrase suggests one doesn't need to choose one because the day when they thought about and decided on the matter is the best day to start acting. The modern interpretation: You should put your decision into action right away!