A fast on Sept 7 is not canonically required, but…
Granting that popes are not held to the same canonical standards as are diocesan bishops in regard to their manner of establishing obligatory days of penance (1983 CIC 331, 1244), I do not think that Pope Francis “proclamation” of September 7 as a day of fast and prayer for peace in Syria is canonically binding on the faithful. The manner of the proclamation used (an Angelus talk) and not used (e.g., publication in the AAS), and of the vocabulary used (e.g., invitation) and not used (e.g., decree, direct) suggests the urgent-but-still-exhortative character of the pope’s plea, not its binding character. Besides, in cases of “doubt of law”, potential obligations are not binding (1983 CIC 14, 18).
In short, a Catholic who does not observe a fast on Sept 7 does not violate canon law. What such disregard for the pope’s unusual request might indicate about one’s desire to act with the Successor of Peter is another question. + + +
Same day update: Fr. Z reaches the same result that I do, here, and has some good suggestions.