Hallmark celebrates all things Christmas with a gusto that is hard to match: Here come five holiday-themed movie premieres, beginning tonight with the Marine-centric rom-com “Christmas on Duty.”

A prologue introduces us to Blair (Janel Parrish, in her 9th Hallmark movie!) and Josh (Parker Young in his Hallmark debut). At the Marine basic training in Quantico, they both scored at the top of the class but upon graduation, it was Blair who got the job Josh wanted.

Jump years later where they meet, banter, fight and are sentenced to work together for 24 hours at Christmas.

For the two stars, there were none of the fabled “chemistry tests” before casting was finalized. They just clicked.

“Luckily,” Young, 37, said, “Janel could have chemistry with a doorknob. Because I’m just barely above a

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