Just a note that the currently accepted answer is deprecated in Rails 3. You should do this instead:
Comment.where(:created_at => @selected_date.beginning_of_day..@selected_date.end_of_day)
Or, if you want to or have to use pure string conditions, you can do:
Comment.where('created_at BETWEEN ? AND ?', @selected_date.beginning_of_day, @selected_date.end_of_day)