I think the words will be spoken by a human operating a telerobot on the surface :). Sorry, I'm not one of those who think that we should go to Mars as a "backup". I think it is a great place to explore but not a place to settle. It's far worse a place to settle than Antarctica or the sea bed or mountain tops or deserts. And we don't settle those places.

See for instance my Ten Reasons NOT To Live On Mars - Great Place To Explore and Why We Can't "Backup Earth" On Mars, The Moon, Or Anywhere Else In Our Solar System

Also I don't think humans will land on Mars, at least in the near future.

I think they will study it from orbit for planetary protection reasons. Even in Antarctica we are careful not to introduce surface life into lake Vostok.

To Explore Mars With Likes Of Occulus Rift & Virtuix Omni - From Mars Capture Orbit, Phobos Or Deimos

One of the main scientific motivations for going to Mars is to search for life, or to find out what happens to a planet like Earth if life doesn't evolve there -  but we now think there are possible habitats for present day life on Mars, at least probable enough to mark out special regions. We don't want to go to Mars just to find life we brought there ourselves. Especially in case of a hand landing.

Instead they would look at it from orbit. Explore from Earth first as we are doing now, then when we have the capability, explore from orbit.

So, then there is no particular moment that counts as arriving at Mars. But there would be a first moment when a human in orbit operates a rover on the surface directly by telerobotics. And what's more then everything would be recorded from the astronaut's telerobot point of view, in stereoscopic vision, haptic feedback so you can feel the rocks yourself - and you could do it with binaural sound also if there are any sounds to be heard on Mars. The wind perhaps and sounds of the machinery.

So - these would be videos that would surely be played over and over, first telepresence video from Mars surface. So it might well be marked by some occasion.

If so I'd be inclined to make wishes for the future, and try to turn it into an auspicious moment.

Something like:

"We are here for all mankind and for our beautiful home planet Earth. And we are here in a spirit of exploration and open ended discovery and also humility, aware that there is much that we don't yet know about our universe. We are a young species, with a long future in front of us, and may we use that well, and may we make good use of the discoveries we make on Mars."