What does the eagle flies on Friday mean?

Roman soldiers earned a 1 denarius a day for their service to Caesar. The denari was a coin worth about 10 head of livestock and had the head of the current Caesar on one side (heads) and a emblem on the reverse (tails). Several of the different Caesar's like Octavian, Augustus, Vespasian, Aureianus chose to use the eagle as their emblem. Payday when all the soldiers received their earnings, they spent the money like a lottery windfall, like any good soldier would. Hence the common phrase "The Eagle flies on Friday"