Terravek publishes standard maintenance windows: Wednesdays 02:00–04:00 UTC for eu-nl-1, Thursdays for eu-de-1. Most weeks we do not use them.
The rule is that any change with a plausible path to customer impact is announced five working days ahead on status.terravek.com and by email to tenancy administrators, whether or not we expect downtime.
That last clause causes internal argument roughly twice a year. Announcing a change that turns out to be invisible looks like crying wolf, and there is a reasonable worry that customers will stop reading the notices.
We do it anyway, for a specific reason: the alternative requires us to correctly predict impact in advance, and we are not that good. The changes that have caused customer impact in our history were, without exception, ones we expected to be routine.
Emergency changes are announced as early as we can, which is sometimes after the fact. Any emergency change that caused impact gets a published post-incident review like any other incident.