Tracker Updated
You'll probably get an email for test with instructions a day or two before March 25th. If you don't receive an email even on the 25th, Email them on the ID mentioned in below FAQ. As for the your query that you have "selecting the desired date" , you can choose any day to give the test, doesn't matter what day it is or what time it is. ( weekend, weekday , 10am , midnight .... blah blah .. you get the point )
https://www.cic.gc.ca/english/helpcentre/answer.asp?qnum=1566&top=5
PS. You don't need email invitation to take the test. Follow this
link . At the bottom , click on sign-in and it would take you through the same website that you get in the email to take the test.