  The recurrent fast radio burst FRB 180916 was recently shown to exhibit a 16
day period (with possible aliasing) in its bursting activity. Given magnetars
as widely considered FRB sources, this period has been attributed to precession
of the magnetar spin axis or the orbit of a binary companion. Here, we make the
simpler connection to a {\it rotational period}, an idea observationally
motivated by the 6.7 hour period of the Galactic magnetar candidate, 1E
161348--5055. We explore three physical mechanisms that could lead to the
creation of ultra long period magnetars: (i) enhanced spin-down due to episodic
mass-loaded charged particle winds (e.g. as may accompany giant flares), (ii)
angular momentum kicks from giant flares and (iii) fallback leading to long
lasting accretion disks. We show that particle winds and fallback accretion can
potentially lead to a sub-set of the magnetar population with ultra long
periods, sufficiently long to accommodate FRB 180916 or 1E 161348--5055. If
confirmed, such periods implicate magnetars in relatively mature states (ages
$1-10$ kyr) and which possessed large internal magnetic fields at birth $B_{\rm
int}\gtrsim 10^{16}$ G. In the low-twist magnetar model for FRBs, such long
period magnetars may dominate FRB production for repeaters at lower
isotropic-equivalent energies and broaden the energy distribution beyond that
expected for a canonical population of magnetars which terminate their magnetic
activity at shorter periods $P \lesssim 10$ s.
