You created ticket at Saturday, in next 24 hours after ticket was created supports accepted your ticket ( it was Sunday ) and told that your class will be changed at Monday ( because we are not changing classes at Sunday ). Right now they are chenging classes based on queue. And as it's telling in topic, your class change must be made in 12 hours after ticket accepted, but can't be changed at Sunday. So count starting from today morning ( when supports start working ).