If the only trusted party is yourself, and you can't guarantee being available when the message contents are to be made public, then what you can do instead is to build a device (physical or virtual) that will automatically make the key public at the required time, and then hide the device.
An easy way would be to buy a virtual server from Amazon or any of hundreds of other companies - perhaps several servers in foreign countries - under a different identity, not traceable to the identity of the person who published the message. Ideally, you would buy this server several years before releasing the message. These servers simply sit and wait, doing nothing (perhaps hosting an innocent-looking email or FTP server), until the specified date, and then publish the decryption key through multiple public channels so as to satisfy your definition of making the information "public."
Nobody would even know that these servers exist, so nobody is looking for them; and their purpose can be sufficiently obfuscated that nobody who stumbles across them by accident realizes what they're for. There are many millions of internet-connected servers - yours are simply lost amongst the noise.
This would be sufficient unless the message is considered by the public to be important enough that there would be a world-wide effort to locate the key, on a scale that would inspire governments to perform sophisticated traffic analysis every virtual and physical server that went online in the past decade, and then manually examine all files and code on each of the (millions of) suspicious ones, looking for hidden information.
In that case, you could hide the device even further. If you really want to do this James Bond style, put the message on a tape connected to a shortwave radio transmitter with a reserve battery in Antarctica (where it might get buried in snow), or a remote Brazil jungle (where it might get damaged by animals), or at the bottom of the ocean, with a chemically-inflated air bag to float it to the surface at the specified date and time (where it might corrode - maybe Lake Superior is safer?), or buried shallowly underground, with a periscope-like antenna.
Of course, the difficulty and cost of any of these options depends on how long you want to keep the device hidden. If it's a century, it's likely that Internet protocols will have changed, and anything more complex than analog short-wave radio might be infeasible. (And it may be that nobody is listening to shortwave anymore, either.) If it's only a few months, your device could simply be a prepaid smartphone connected to an external battery pack and dropped someplace moderately obscure. There are already lots of cell-based remote sensors on the market, that automatically make a phone call or a web connection when some criteria are met, so this would be almost undetectable - it would look to the cell phone company like just another one of these increasingly ubiquitous devices.