If you're like my family and have an older Kindle, you need to do an update — today. (My son uses a 2011 hand-me-down Kindle, so I know what we'll be doing.)
Amazon has a Critical Software Update for Kindle E-Readers on its website with an urgent warning: "This update needs to be completed by March 22, 2016."
If you do not update your device by March 22, 2016, you will receive the following message on your device: Your Kindle is unable to connect at this time. Please make sure you are within wireless range and try again. If the problem persists, please restart your Kindle from the Menu in Settings and try again. If you see this message on your device on or after March 22, you will need to manually install the latest software update for your device.
It will be possible to update an older Kindle after March 22, but it will be much more difficult. Instead of updating the device when it's connected to the Internet over a wireless or Wi-Fi connection, you'll have to do it manually with your computer and a USB cable. Instructions for how to update without connectivity can be found on Amazon's Fire & Kindle Software Update page. Click on the appropriate model for details.
The Kindles that are affected by this critical update are ones released between 2007 and 2012. Newer Kindle models update automatically. Here's the chart from Amazon's website that shows which models need updating:
Even if your Kindle is sitting around collecting dust because you're reading your Kindle books through an app on a different tablet, it's smart to charge it up and do the update today. At some point you may want to use it again or hand it down to someone, and then you'll have to go through the hassle of updating it manually.