This alert was posted on the Inside AdWords blog a short time ago:
AdWords system maintenance on June 7th
On Saturday, June 7th, the AdWords system will be unavailable from approximately 10 a.m. to 2 p.m. PDT, for maintenance. While you won't be able to sign in to your accounts during this time, your campaigns will continue to run as usual.
AdWords system maintenance typically occurs on the second Saturday of each month during the above times -- with this month being an exception to that rule.
In the interest of giving you advance notice, there may also be maintenance on Saturday, June 21st. However, we will confirm before that date.
We'll continue to update you via the blog as we always have, but please make note of the June 7th date, the tentative June 21st date, and our scheduled maintenance further down the road.