From the developer's description:
WA Scheduler periodically starts and stops WhatsApp to check for new messages. You only need to set the days and hours you want sync in, then select an update frequency. By running in background you can get message notifications from WhatsApp without being always connected.
Some screens, again courtesy of the developer:
Configuring WhatsApp Scheduler, setting WhatsApp for daytime operation on workdays only....
There's a full changelog for recent versions and a download link here.
This could turn into a bit of a FAQ for this and future news and 'flow' items on AAS, but in order to install this you'll need to be running custom firmware and with the InstallServer patch active (as far as I know - clarifications welcome in the comments), or similar. If you're not sure if your device can install 'unsigned' applications then you almost certainly can't!
If you're a newbie to custom firmwares, check out some of my Delight adventures, e.g. here.
Also, WhatsApp itself isn't in the (frozen) Nokia Store, so download it here.