  We systematically search for quasiperiodic oscillatory (QPO) signals on the
month timescale among the 1525 sources given in the Fermi Large Area Telescope
Light Curve Repository. We find a transient QPO of 31.3$\pm$1.8 days in the
gamma-ray band light curve of the TeV blazar S5 0716+714, which has seven
cycles (MJD 55918-56137) for the first time by weighted wavelet Z-transform and
Lomb-Scargle periodogram methods. Monte Carlo simulations based on the power
spectral density and probability distribution function were used to evaluate
the confidence level of the QPO, and the result is $\sim 4.1\sigma$. Seasonal
autoregressive integrated moving average modeling of the light curve revealed
it is a significant physical QPO. The physical models to explain the sporadic
QPO of the month-timescale QPOs in blazar were discussed. Our studies indicate
that the helical jet model and blob move helically in a curved jet model to
properly explain this kind of transient QPO.
