What will happen if you 'don't' download it before the 24th is it won't be free, but having downloaded it and installed it then it should be free for the duration you have it. If however, you come to upgrade it then I assume you would pay.
Many of these free products are used as a hook, you like it, a new version comes out and you upgrade, paying for the upgrade. Some offer a free copy of an earlier version and not the current version to hopefully get you to upgrade.
I would say it differs from the download of the in that this offer extends for much more than a day, but the principle is the same, a free application for a limited period.