A Wikimedia Hackathon...

... is an unconference

which means that the schedule is made up very spontaneously, the event is focused on hands-on work, training, and technical discussions.

.... with around 150 to 300 attendees

from all over the world, about 30% WMF developer employees and 70% volunteer developers. The number of participants can be anything between 100 and 400 people, depending on the location and the organizers' vision of the event.

... that takes place over 3 days in spring

The yearly Wikimedia hackathon is three days long: Friday through Sunday or Saturday through Monday.
The yearly Wikimedia hackathon is generally held in the spring (March, April or May). If you are considering a date in May, make sure that it is not too close to that year's Wikimania which also has a hackathon with many of the same participants.
Smaller hackathons can be longer, shorter, midweek or over the weekend or at a different time of year.

... with a typical schedule

Day 0: Event organizers arrive and set up. Participants begin arriving in evening. Low key social event or organized group outings. Pre-registration and name badges

Day 1: Registration, All-hands/welcome/intro, Sessions start, hacking, mixer/social event, hack through the night option

Day 2: Sessions and hacking continue, hack through the night option, social event

Day 3: Sessions and hacking continue, wrap up, closing social event or dinner, some participants leave but most stay the night.

Day 4: Everyone heads home, organizers must still be onsite to help participants.

... each year in a different location

Run in a different location every year, with different local organizers supported by the Wikimedia Foundation's Developer Relations team and experienced volunteers.

Ideally the Wikimedia Hackathon model can be replicated and adapted to different regions. Hackathons are welcome especially in Wikimedia and developer hubs like India, North America, Latin America, Greater China, Russia... Wikimedia chapters are encouraged to play a key role organizing events in their territories, but the invitation is extended to all Wikimedia formal and informal groups. A team of three dedicated people playing well their cards can organize a successful hackathon.

Wikimedia organizations are strongly encouraged to support hackathons organized by others, sponsoring travel for technical contributors active in their communities.

... at a suitable venue

Fast and reliable wifi (this is very important)
A room large enough to hold all attendees for intro/wrap up. This space should have a stage, a professional projection system, microphones
A large working space for hacking (this can double as the introduction room or can be another large room, lounge area of a hostel, etc)
Multiple break out rooms for sessions, talks and focused hacking.
Open 24 hours a day during the hackathon
Lodging/ accommodation for participants onsite or very close by
Close to an international airport with an easy transfer options at all hours

... cares about everyone having a good experience

Goals are measured by participants' satisfaction, on-boarding of newcomers and tangible outcome. The number of attendees is not a goal.

... welcome newcomers

Hackathons are good opportunities to reach out to new contributors. Specific activities must be planned in order to introduce them to Wikimedia tech and the activities of the event.

... showcase demo-able projects

We focus our hackathons on teaming, prototyping, and a showcase at the end of the event. A hackathon provides a good environment to accelerate the development of new ideas, to collaborate on tough problems, and to identify contributors capable of leading independent projects. The Wikimedia Foundation will look at the projects showcased, seeking opportunities to support the productization of interesting concepts.

... is organized by a dedicated team

Some of these roles can be held by the same person, but usually a team of at least three local people and two people from the WMF team is needed:

Project lead from hosting chapter/organization
Project lead from WMF
Logistics (venue/accommodation)
Finances
Scholarships (Co-owned by WMF and local group)
Visa support
Travel support
Communications
Outreach/fundraising/engage local communities

... is supported by the WMF

financially with a budget, and personally with a dedicated events team from the Developer Relations team.

Budget for the organization approved by the Developer Relations and complemented by travel sponsorship support from the Wikimedia Foundation, the local organizers, and other organizations.

Local organizers focus on local work. Global aspects like travel sponsorships and the program can be delegated to the community. The WMF helps where is needed.