The DebConf registration team has the hard job of coordinating accommodation, food, and payments for hundreds of people.
There are two major dates here: the sponsored registration deadline, and the reconfirmation deadline. There is also the date of opening registration, but that's usually more a matter of "how soon can we?" than "when should we?"
The sponsored registration deadline (April 15 for DC10) is the deadline for requesting sponsored accommodation, food, or travel. This allows us to figure out how much we'll have to pay for attendees - attendees who register after this point should pay enough to cover their own costs.
The reconfirmation deadline (June 10th for DC10) is used as a ping for attendees, particularly the sponsored ones. Attendees must reconfirm they will attend, so that we don't waste money reserving more food/rooms than we will have people using them. This deadline usually depends on the date we must give firm numbers to our accommodations.
Historically, registration hasn't ever closed, anyone can come, just if you register too late you won't necessarily get any free stuff (and definitely no free food/accommodation). The deadline for food and accommodation themselves has been based on the logistics of the conference itself - DC9, when we ran out of physical rooms for accommodation, and DC10, when we had to give firm housing numbers to Columbia. Food has tended to be easier as caterers could take updates on shorter notice, usually the day before.
DebConf8 began a system where attendees could pay a registration fee if they desired. This was encouraged, for example, for attendees whom could get this fee reimbursed by their company, but was not required nor encouraged of sponsored attendees. DC8-9 had two classes of fees: Professional, covering the actual costs, and Corporate, covering about double of professional and for companies or registrants whom could provide extra financial support for DebConf. DebConf10 added an extra layer of "pay per day" on top of this.
It's up to the registration team to pin down attendee numbers as early as possible, as this is usually the biggest cost. DebConf provides food and accommodations to a huge number of people, but the numbers are constantly changing. It's hard to know how many people will drop out at the last minute. It's hard to know how many people will actually pay a corporate or professional fee, as opposed to mistakingly selecting it when they'd rather something else. Our registration system can't support all the little intricacies of how much an attendee will cost us, or pay us. Without knowing how many people will attend, how can we accurately get the right number of meals and right number of rooms? Sponsored people may always choose to not eat some meals with us, possibly saving us money there. How many people with travel sponsorship will actually request their full reimbursement?
The registration system, Pentabarf, is used to track attendance numbers, but it isn't completely reliable. Not everyone in it will attend all days, and most people who aren't staying with us don't have accurate dates of attendance. Thus, it can only be used as estimates, not for all decisions.
The difficulties above are challenges for the registration and budgeting teams, but are not too bad - they just require thought, time, and answering many questions from attendees.