Depending on how you run your selection process and/or national debating tournament, you may wish to hold events on just one weekend or several. It is typically easier to have events on weekends if people need to travel however, rounds of debate, as part of a competition or regional trial, can take place during the school day, especially if a school is hosting it. Asking a competing school to host any event like this may be cheaper than hiring an independent venue. You should consult with the schools and individuals who are involved to work out what will be the most convenient time and date and try to avoid clashing with any other major similar events in the calendar which may mean your debaters and/or judges are occupied elsewhere.
Generally, your trials should take place at a central location and most nations tend to opt for their capital city because transport links and accommodation are often the most convenient there. However if your organising committee or your main debate organisation have a base in a particular place which is not in the capital, it may be worth holding your selection events there instead as you may have more personnel and resources readily available.
The World Championships take place either in January/February or July/August of any given year. There have been a few exceptions to the rule, but they are always voted on and made public very early on. Bearing this in mind, and the fact that team registration usually closes a few months before the tournament itself, you probably want to hold your selections no later than three months before the tournament that year. That said, some nations choose teams well before then and some after. You can only start working on prepared cases two months before the tournament, once the prepared motions are released, but you’ll have to be registered before then and many teams will want to start working on general debate strategies before hand. There is no right or wrong answer here but you just need to figure out what works best for you and your nation and try to maximize the time you do have to the full
If it is necessary to provide accommodation for debaters during a selection event, you will need to investigate the cheapest and more convenience way to do this as early as possible. In many cases, you can ask the schools to make their own arrangements for their own students and only offer to help those who have a particular need. Youth hostels and home stays can often be good options.