It looks like the keyword you provided — — appears to be a file naming convention, possibly from a personal video archive, a torrent metadata string, or a labeled digital recording.
But if you're strictly asking for the correct article for the given title as written (keeping the original word order), the article is already present and correct: (not "An"), because "day" begins with a consonant sound. a day of sailing naturist 52m20s avi007 15
For now, here’s a short outline of what a paper on “A Day of Sailing Naturist” could cover if based on a typical naturist sailing video: It looks like the keyword you provided —
: Known for a laid-back "come as you are" attitude, the waters around and possibly from a personal video archive