Action Details
Requirements and how-to articles for Steam's action:

Coming Soon.
If you are going to be hosting a limited time event within your game, you should post about it on your game's event page to make sure players are aware of it.
Coming Soon

Currently in Open Beta

You need this to get it done:

To create a new event, you'll first be asked to select a category and sub-category. This impacts where and how the event shows up, and also helps players understand the event's functional context.
Event Title
The title of your event. It should catch the user's attention and let them know what the event is about. (e.g. "Winter Event - Limited Time Skins Available!")
Event Subtitle/Summary (optional)
A short summary of your event. (e.g. "The new Winter PVE event begins next week! Will you help defend the realm?")
Event Cover Image
The cover image asset is used to represent your event/announcement in a number of places across Steam, including on the store page for your game and in the Steam Client Library. It's used to link to your event/announcement and may be displayed alongside content from multiple games, so the image should be clear and recognisable.
Dimensions: 800 x 450 pixels (Note: Valve has included an image resizer tool in their workflow, so you don't have to worry about getting the image dimensions exactly right.)
Event Description/Body
Users can interact with the event body section of your announcement. This will mostly be comprised of text (with BBCode formatting available) which describes your event in detail but it is highly recommended to also include images and widgets.
Embedded Image (optional)
You can upload images to be hosted on Valve's servers, or you can link to images you have hosted elsewhere. Whilst this is optional, it is recommended to have at least one image to help grab the user's attention whilst scrolling through their activity feed/library/etc.
File format: .jpg/.png/.gif
Link Widget(s) (optional)
When you include a link to specific websites, the page will automatically turn them into a preview of that link. Valid links include YouTube videos, Steam Workshop files (or other shared files on Steam such as screenshots, guides, or artwork), Twitter posts and links to another game's Steam store page.
Youtube Video Embed (optional)
You can insert one or more YouTube videos and specify how you'd like them to be presented on the page.
Start and End Dates
The start and end dates for your event. (Having both a start and end date allows Valve to properly archive events after they are over.) Note that you cannot publish an event with the start time set in the past, and the end date can be edited to either end the event immediately or extend it.
Localisation (optional)
When you post an announcement, the default language will be set to English. Users with Steam set to other languages will see the English version of the announcement. It's a good idea to include localisation for all other popular languages that your players are using.
HTML Import
If you prefer to use HTML, or have an existing HTML version of an announcement, you can use the HTML import option. This will convert your existing document to use BBCode, which is supported by Valve's announcement system.
Event Spotlight Image
A spotlight image can optionally be included to prominently display your event or announcement to all of your users. The image will sit near the top and span the full width of the game details page. If a user clicks on your spotlight image, they will view the Event/Announcement Detail Page. The spotlight image should focus on the content of your event and should include a call to action for the user.
See Howto articles for image size template.
Requirements missing or found errors? Report

How-To Articles
A list of useful links for you:

Steamworks Documentation - Events and Announcements Tools
Information about Steam's new expanded Events and Announcements system.
Steamworks Documentation - Events and Announcements Examples
Examples of Events and Announcements which follow the new format.
Link missing here or found errors? Improve How-Tos