There could be many reasons why a function is not held on the "correct" day. As noted in another note, there might be a space limit in the Synagogue. In 1964 I was working in Paris and the only time for those 8 months when there were more than 2 young people reading their pieces was when there were twins or triplets in the family.  The place was always packed.  But when we turned to New Year, the resident congregation took over a Concert Hall because their Synagogue could only hold about 10% of the congregation and they allowed an American group from all over Europe to have their services in that building.  Likewise, at present some families have delayed Bar and Bat Mitzvah until after the end of Covid restrictions (not only of allowed numbers in the congregation but also on travel from other countries I would imagine that damage from Volcanos and bombs could also require changes of venue or of day .

David Harrison,

Birmingham, England