  $\xi^1$ CMa is a monoperiodically pulsating, magnetic $\beta$ Cep star with
magnetospheric X-ray emission which, uniquely amongst magnetic stars, is
clearly modulated with the star's pulsation period. The rotational period
$P_{\rm rot}$ has yet to be identified, with multiple competing claims in the
literature. We present an analysis of a large ESPaDOnS dataset with a 9-year
baseline. The longitudinal magnetic field $\langle B_{\rm Z}\rangle$ shows a
significant annual variation, suggesting that $P_{\rm rot}$ is at least on the
order of decades. The possibility that the star's H$\alpha$ emission originates
around a classical Be companion star is explored and rejected based upon VLTI
AMBER and PIONIER interferometry, indicating that the emission must instead
originate in the star's magnetosphere and should therefore also be modulated
with $P_{\rm rot}$. Period analysis of H$\alpha$ equivalent widths measured
from ESPaDOnS and CORALIE spectra indicates $P_{\rm rot} > 30$ yr. All evidence
thus supports that $\xi^1$ CMa is a very slowly rotating magnetic star hosting
a dynamical magnetosphere. H$\alpha$ also shows evidence for modulation with
the pulsation period, a phenomenon which we show cannot be explained by
variability of the underlying photospheric line profile, i.e. it may reflect
changes in the quantity and distribution of magnetically confined plasma in the
circumstellar environment. In comparison to other magnetic stars with similar
stellar properties, $\xi^1$ CMa is by far the most slowly rotating magnetic
B-type star, is the only slowly rotating B-type star with a magnetosphere
detectable in H$\alpha$ (and thus, the coolest star with an optically
detectable dynamical magnetosphere), and is the only known early-type magnetic
star with H$\alpha$ emission modulated by both pulsation and rotation.
