It all depends upon what "Type"of Morris you are talking about.
Cotswold (South Midlands) Morris was perform at Whitsun - because that was the only time in the year the dancers had time off from farm work (also a quiet farming time) to dance.
Border Morris was performed over the Christmas period, as was Sword Dancing (Long & Rapper).
Molly Dancing was around the old New Year.
Sharp's meeting with Headington only happened because the Headington men, nearly all builders or masons, were frozen out of work, and because the Carol singers were out collecting, they thought it a good idea to dance and to collect. Sharp just happened to be visiting his Mother in law at that time.

Tim Radford