What new practice did the pigs require of animals before entering the barn for Sunday meetings in Animal Farm?

I think that the answer you are looking for is found in
Chapter 3.  In that chapter, we are told about what the animals did every Sunday after
the Revolution.

We are told that they got to sleep in --
breakfast was an hour later than usual.  Then they went out into the farmhouse garden
and had a flag raising ceremony.  After that, they all went in to the meeting.  At these
meetings, they would discuss issues and make plans for the next
week.

This only lasts until Chapter 5, though.  Then
Napoleon announces that there will no longer be any meetings.  At that point, though,
there is a new thing which may actually be what you are talking about.  They have to
file past the skull of Old Major now before they go to hear their orders for the
week.