Both your blog and your RSS feed use some form of this Movable Type template tag:
lastn=15 means that it will sort in chronological order with the newest on the top, and it will only show the latest 15 entries. But sometimes you want something to sit at the top for a while, or you want to "post-date" your entry since it's an event announcement that doesn't expire until the day of the event. Most people in these cases will get around Movable Type's chronological sort by changing the "Publish Date" of the entry for some date in the future. In UThink, in a new "Create Entry" form, on the right-hand side, you should find a "Publish Date" field where you can change the date.
There are alternatives to the "lastn" sort method: http://www.movabletype.org/documentation/appendices/tags/entries.html#sort_by
One really interesting and untested way to sort is by "field:BASENAME". In lieu of "BASENAME", you would enter the basename of a field (or custom field). Like I said, I haven't tested this, but I would imagine it would be alphanumeric order where the letters and numbers have been converted to a hexadecimal number. That's just a guess.