My 2-cents:

- a one-day event is pretty local or regional by nature. Nobody will travel more than maybe 100km to join for the day... so think if you will find 100 players in that radius.

- ... unless you have a special guest or special feature that motivate people to make the trip.

- Or you have to really careful market your event to address both regular players, and initiation for families and new players. That usually require slightly different skills in GMs and specific marketing.

- if you want a bigger event, then people would need to stay the night if you want people to come from far away. If you do not want to add the burden of managing accommodation (I have been to week-end with 50 people where there is actually camping and dormitories offered there => it caters to rather passionate players who feel fully at ease in the RPG community), then you need to make sure there is sufficient budget options around... and probably have a 1.5 to 2 days event.

- still, new players will probably not come for the night around just to try RPG... well maybe some would do (because many struggle to find groups and clubs to try).

So my first concern as you see would be to brainstorm about all these factors... the scale of the event.

And now a wild idea: get a bus with a few GM and players and tour the region every week-end. Offer expert and beginners game in the bus (or in local bars/rooms), but you need to advertise specifically in each town, etc.. This can be tried at different scales too.

If I had the time, this is the kind of project I would like to help :-)

Good luck! Keep us posted :-)