The best way to find out about the IELTS dates is to visit the British council calendar. Different cities have different dates, so you will have to look for your city and find out that which date is available. You cannot the IELTS exam when you want to take it, the exam is offered on certain days and you will only be allowed to sit on the certain day. However, you may change the date of your exam if you are encountered by an emergency.
You should visit the IELTS website to study the terms and conditions relating to date changing process.