Using once just adds emphasis to the fact that it was indeed only once. I visit my parents every 5 years. I visit my parents once every 5 years.

I celebrate Thanksgiving every year. (Yes.) I celebrate Thanksgiving once every year. (No. This doesn't really make sense, since Thanksgiving only occurs once every year) I go to church every year. (This means that I go to church at least once every year.) I go to church once/twice every year. (This means that I go to church exactly once/twice every year.) once every year once per year once a year These three are similar, with "every" emphasizing the regularity - i.e. you don't miss any years.

The two phrases do have the same basic meaning. However, the first has more emphasis. "Once every week, I see my friend." In this sentence, the time and frequency are emphasized. "Every week I see my friend." In this sentence, the time and frequency are less important. Note: this only works when it is "once." "Twice every week, I see my friend," does not put any special emphasis on the frequency. Also note that we tend to prefer having one part of the equation be singular. "Once every three years" works, and "three times every year" works, but "twice every three years" sounds very awkward.