Something changed. On the event page, open Actions and choose Send update, say what changed in your own words, and press Send.
An update goes to the people who said yes and the people who answered Not sure yet. People who said no, and people who never answered, don't get it.
If the date or the place moved, tick Ask everyone again. Then the people who said no get it too, and everyone it goes to is asked whether they can still make it. People who never answered still aren't sent anything.
The people who had said yes keep showing as yes, marked asked again, so your count never quietly drops while you wait for them.
Changing the date or the place in Edit sends nothing by itself. When you save, a box reads Tell the 12 people what changed; leave it ticked and you go straight to the update with the re-ask ticked.
An update by email goes at once, whatever the hour.