According to Google AdSense blog, this Saturday, the Google AdSense engineers will be performing routine site maintenance from 10am to 2pm PDT. You won’t be able to log in to your account during this time, but your ad targeting won’t be affected, and they’ll continue to record your clicks, impressions, and earnings as usual.

They’ve provided the maintenance start time in a few other cities for our international readers:

London – 5pm Saturday
Kiev – 7pm Saturday
Mumbai – 10:30 pm Saturday
Singapore – 1am Sunday
Sydney – 4am Sunday

I think this maintainance has to do with the introduction of interest-based ads.