Starting Friday 10/16 (tomorrow) at 2pm PDT, sign-ups for new Developer Edition orgs will be down while we undergo maintenance as part of the Winter '16 release. If you anticipate needing a new DE this weekend we encourage you to sign up for it now. You will not be able to sign up for a new Developer Edition while this maintenance is in process. We will be sharing updates on the maintenance window via the @SalesforceDevs twitter account. 

For more information, please see this blog post: https://developer.salesforce.com/blogs/developer-relations/2015/10/upcoming-maintenance-on-developer-edition-sign-up.html