From Longman Dictionary of Contemporary Englishbe somebody’s lucky daybe somebody’s lucky dayspoken used to say that something good and often unexpected will happen to someoneWe’re going to win. I just know it’s our lucky day →lucky
Examples from the Corpus
be somebody’s lucky day• "Look at the size of the fish I caught!" "It must be your lucky day!"• Anyway, that day was obviously a lucky day.• But it was Swindon's lucky day.• It was not her lucky day.• Read the stars in the magazines and the paper: Todayis your lucky day.• Friday used to be considered a lucky day for weddings in Gerrnany.