During the holding procedure is it required that the pax must have the seat belt on?
There are no regulations which require passengers to be seated when the plane is flying a holding pattern, but there are a few practical advantages to keep the seat belt sign on during this phase:


The plane is flying at a lower altitude, where it is more likely to encounter turbulence and other unstable weather conditions. Up in FL330 it is usually clear skies all along.
The plane may need to get down quickly after an approach clearance is issued (after all, it is busy airspace). From that point onward the pilots may deploy the spoilers to lose altitude (which shakes up the plane a little), turn towards the approach course and without you noticing, landing gear is down and locked already. At the same time, we know it takes quite a few minutes after the seat belt sign has been turned on until everyone is seated (passengers won't just rush back to their seat after hearing the Ding Dong tune). If the seat belt sign is not illuminated in this period, there may not be enough time for the cabin crew to prepare the cabin for landing.
It reminds the passengers that if they wish to get out of their seat for whatever reason, they should make it quick.


If the flight deck tells the passengers it is going to be a delay, the cabin crew is usually a bit relaxed at enforcing the seat belt sign. E.g. if someone is going to the bathroom, they won't block unless that person is hanging around standing. When the approach clearance is given, another announcement will be made, and the cabin crew will now make sure everyone is seated because landing will commence shortly.