[eluser]kurucu[/eluser]
- Could offer drag and drop between two columns: can and can't
- Allow them to type in the weekend date (or a date) and show/select weekends near it
- As you suggest (checkboxes) but arrange weekends into rows for each month, so a whole year can be shown and navigated easily
- select all in x month, select all other ones, select all in manually typed range and so on.
- allow upload of iCal/other file and pre-select busy weekends
- perhaps a view which works when the window is narrow, so that they can scan between their calendar and your page easily
- remember dates between years e.g. anniversaries and birthdays (that's a feature, not a UI concept)
OR
- a mixture of the above (including none!)
Whatever you do, remember to keep things logical and simple. Dates are presented in a certain way (particularly in order of time, but as weeks/months/highlighted weekends etc) which you should stick to. Use simple, obvious controls and usual etiquette.
Edit: If they are likely to select many, don't force the page to refresh between selections because it'll become tiresome. Offer a big form or ajax selection.