The seventh day of the seventh month seems to be significant in the mind of our theoligian/liturgist Pope. Two years ago it was the publication of Summorum Pontificum, today it is the publication of his long awaited encyclical Charity in Truth.
I am sure there is a significance in the 7th of the 7th, the Pope is concerned about symbolism and has written about "sacred time". Seven is the number of signifying perfection or completion. The seventh day is last day of Creation, the day on which God rested, according to the rabbinic tradition to enjoy all he done. Possibly it is one of those little flashes of papal humour saying, "I have done the work, now is the time for rest, its holiday time".
I suspect we will see in the encyclical something of a milestone in this pontificate, a seed that is planted that will come to fruit in decades to come, just as the Motu Proprio will prove to be.