As documented in the Demo and CalendarDateSelectParameters:

This disables all dates after the beginning of today: <%= calendar_date_select_tag "birth_day", "", :valid_date_check => "date < (new Date()).stripTime()" %>

This disables all dates before the beginning of today: <%= calendar_date_select_tag "birth_day", "", :valid_date_check => "date.stripTime() > (new Date()).stripTime()" %>

Use can use this to disable weekends, holidays, anything, really. Be creative :)