Muster of Rohan was the term describing when all of the forces of Rohan were brought together. Prior to the Battle of the Pelennor Fields, the Muster of Rohan took place at Dunharrow.

"Riders of Rohan! Oaths you have taken, now to fulfill them all. To lord and land!"
Eomer before riding out to Minas Tirith, in the 3rd film
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