Why is this necessary? Because Freeview is making technical preparations for the introduction of Freeview HD later this year, and to make Channel Five as widely available as the other terrestrial channels at the time of digital switchover.
Ater retuning, more than 500,000 homes should be able to receive Channel Five for the first time. However, some viewers who receive their Freeview service from a relay transmitter will no longer be able to receive ITV3 and ITV4.
Some BBC radio stations will also be affected. Listeners in Border, the West Country and parts of Wales will lose those services until they retune their Freeview equipment.
At the same time, a new channel called Quest will launch on Freeview channel 38.
Viewers using a Freeview TV or set-top box should retune from lunchtime onwards on September 30th. You may already have seen a message advising you to do this on your TV screen.
If you wish to remove the reminder message from the screen, press the yellow button on your remote control. Otherwise the message will disappear automatically after one minute.
It's possible a limited number of early Freeview TVs and set-top boxes may fail when retuned. To find a list of models affected, contact Freeview on 08701 111 270 or click here.
There's plenty more advice about the retuning programme on Freeview's website, and specific instructions on how to retune your TV or set-top box here.