Boxing Day is the traditional day to sow onions.
“Sow just after the shortest day, harvest just after the longest day”
is what you’d hear from the old boys on the allotment site. There is also a theory that it’s a handy excuse to escape the family and visitors of Christmas with a visit to the allotment.
If you don’t have the facility to keep the seedlings and then plants in good light and warmth until they go into the open ground during May/June then there is no reason to sow so early.