DNS Flag Day is February 1, 2019--that's just two weeks one week from today. If your domain names use DNS servers that aren't ready, they'll be slow or entirely unavailable to a large percentage of the internet. If you have multiple domain names with the same nameservers, you only need to test one in the group. For example, if half of your domains use GoDaddy's nameservers and another half use Dynadot's, you only need to test two domains, one for each provider. Test your DNS provider's configuration here (simple) or here (technical). Edit: Note that if these tests report "minor problems," there's no need for concern unless you operate the nameservers in question. Problems categorized as minor shouldn't cause immediate problems on Feb 1, but aren't ideal long-term. For example, in some cases, they might make it easier for attackers to abuse the affected nameservers.