The short version
For spoilers relating to an upcoming season in general, be it meta or sketches or screenshots, use the spoiler:s03 tag (for season 3 in this example).
For spoilers relating to an upcoming episode, use the spoiler:s03e01 tag (for season 3, episode 1 in this example).
We will be making these spoilers site-wide by default (though you can unhide them in your settings), with episode spoilers being unhidden by default around a day or two after the episode airs. Seasonal spoilers will be unhidden after the first episode airs.
Our spoiler policy is basically to help ensure that nobody gets spoiled by content on the site. While on other sites there's often just a 'spoiler' tag we're being a little more specific with our tags to help people filter with more granularity.
The spoiler tag names are based on convention, which is to address seasons and episode by number, preceded by s or e as appropriate. For instance, season 3 episode 5 would be s03e05.
Seasonal spoiler tags omitting the episode number can be used for more general spoilers that can't be directly related to an episode. These will be site-wide revealed shortly after the first episode is aired. Ideally, season tags should be replaced with episode tags when possible.
If in doubt, add spoiler tags - even marginal spoilers should be appropriately tagged.