Choose the date that your party will take place, and an appropriate time (including which time zone your party will be in.) Make it as accessible as you can by thinking about your target audience - find out when they are usually online to ensure the maximum amount of people will be able to attend.
To get an idea of how many people are interested, try asking your fans to rsvp to the event. You can do this by creating a twitter hashtag that relates to you or the title of your book. People can respond and it will be promoting your party at the same time.
There is a bad side, however. If a lot of people reply back that they can't make it, it might reflect badly on your party and it's reputation.