The cutoff is the 3rd Wednesday of the month, which is the 18th. It should be available on MUIR some time on the 19th, probably mid to late afternoon. Last month the process fell through the cracks, hopefully whatever caused that to happen has been addressed.
Under the rules, unless otherwise announced players should expect you to use the official ratings list as of the starting date of the event, so a February 28th event would normally use the February list. If you’re going to use a different list or use unofficial ratings, you should let players know that in advance publicity, as that could affect their decision as to whether or not to enter your event.