Because kidnapping people and forcing them to act out your interesting make-believe worlds is technically illegal.
It seems there was a mistake with the date in the message from Write World about the flash mob.
The message should have said, Friday April 13th.
Hello! We’re undertaking to coordinate an online flash mob for three very special writing blogs. We have asked three of our followers to nominate one writing blog each to be the subjects of the flash mob, and on Friday, April 13th, we will ask our followers to go to one or more of the nominated blogs and post encouragement, comments, and critiques of the blogger’s creative writing posts. But we need your help to spread the word! Will you join us by reblogging our call to action on the 13th?