Xero are improving their infrastructure platform
As Xero continues to grow their customer base, they are shifting the core cloud platform to Amazon Web Services (AWS), which will help them continually improve the technology that helps Xero function. "It is designed to allow growth to millions of customers and reduce the chance of downtime." - Duncan Ritchie (Xero)
The migration should happen at the least busy times, which is Sunday morning NZ time.You will receive a message a few days before the organization is due to be moved and again 15 minutes prior to it going offline for hopefully a very short period of time.
Read more about the migration here