30 19 * * * /usr/bin/sqlite3 /etc/pihole/gravity.db "update 'adlist' set enabled = 1 where comment = 'Facebook'" ; /usr/local/bin/pihole restartdns 2>&1
0 21 * * * /usr/bin/sqlite3 /etc/pihole/gravity.db "update 'adlist' set enabled = 0 where comment = 'Facebook'" ; /usr/local/bin/pihole restartdns 2>&1
OK, this works.
Note: I also have to schedule the wifi radio to turn of and on (as in, "/etc/init.d/network restart") over in my OpenWRT router at the same time, to force the wifi clients to renew their DHCP leases. This will force a flush of the DNS queries cached by Operating Systems like Linux and Android. Then a Browser page refresh (not just tapping the "Reload" button in Firefox or Chrome) on the clients will give results as expected.