Decide on the details of the game night, and put these somewhere where they can easily be seen. This will include what night you are holding the event, where and what time it begins. You should also be clear on what is happening - what type of games will be available, and who can play? If it's physical games, is it just for set teams, or can anyone play? Are parents allowed to join in? Can siblings come along? Decide on all the facts beforehand, so all promotional material matches up.
Design a poster. You could do this yourself, devote some lesson time to creating a design in class, ask a group of students to make a design, set designing a poster as homework and vote for the favorite or ask a printing company to make a design. The design needs to include all the relevant information you decided on before.
Distribute the posters. Where you put them will depend on who is invited. If it just your class, display them around the classroom. If the whole school can attend, display them throughout classrooms and any social gathering areas, such as dinner halls. You could also display posters in the local community, such as on shop notice boards.
Include a note about the event in the school newsletter, if there is one. This is a useful way of reaching parents, in particular. If your school uses home/school diaries, you could also leave a note in this giving some details.
Create a post for the school's website with information about the event. If your school has an online calendar, make sure the event is listed there. If your school has a list of parent's email addresses, and permission to contact parents, you could send a group email about the event.
Select a group of children to give presentations to other classes and talk to children on the playground, if other classes are invited. If other schools are invited, you could take a group of students to talk to the other school, and hand out some posters. Remember that if you are inviting other schools, you need to give them as much notice as possible.
Display a banner in the reception area or on the playground during the week of the event. This will ensure people know about the event, and give you maximum last-minute exposure.