Long story short: The Bay City Post will not promote private events.
If you have an event in Bay City use the Second Life website to publish and promote that event. The event will be available on the website and in-world using the search tool. It will also place a mark on the map in-world. It has been proven to be quite effective!
To enter an event you need to be owner of the land that must be larger as 512 sqm, you have set a date and time, you know the duration, have a good description and know the category of the event.
The event listing will use the image that has been set using land properties. The landmark used will be in the middle of the selected land so be sure to set a landing point and enable teleporting route in land properties.
A good manual can be found here.
If the event is of interest to the Bay City community you are ok to make a one-time publication in the Bay City Alliance group. Prevent that others might feel it is spam.
The calendar that is visible on the Bay City Post is slightly broken, new events can not be posted there. The calendar only lists Bay City Alliance events and events at The Pen. They were entered a decade ago and nobody knows how to change dates or times.
The Bay City Post will only promote Bay City Events that are part of the Bay City script. The Bay City Post might report about an event after it has happened if it is of interest to the Bay City community.