It's flabbergasting how annoying and difficult some words can be, for a non English speaker like me, when you try to learn how to differentiate them in your mind. I have this silly doubt about these two words which are similar in the writing but their meanings are utterly and completely different. If I'm not mistaken, "skies" is the plural form of "sky", meaning "cielo", "firmamento", in Spanish. And, if I'm right again, the word "skis" is the third person singular of the present tense of the verb "to ski", and its meaning in Spanish is "esquiar". Then, although they are very similar, those words mean completely different things. Therefore, the following sentence would be right like this: "He skis under the blue skies of France at least once a month".