Entries on your journal are posted in the order of their assigned timestamps, starting with the most recent. To keep an entry at the top of your journal, assign it a timestamp far in the future. You will need to select the "entry is backdated" option for that entry in order to post current entries normally afterward. If this option is not selected for the future-dated entry, you will receive an incorrect time value error when posting current entries.
In communities, entries display in the order in which they were posted. Assigning a future date to a community entry will not keep it at the top of the community journal. To post information at the top of a community journal, or as an alternative to posting a future-dated entry in a personal journal, the journal's layout can be customized to include the desired information.
Some S2 layouts include a Blurb or About this Journal section that can be used for this purpose. For other layouts, [something about overrides / custom CSS / custom styles and which options are available to whom, which feels too complicated to write at 3AM and should be written by a Style person anyway].
Posting a 'Friends Only' Announcement
[I'm going to bed.]
9/04: I swear I have not forgotten this and will return to it.