I work as a patent examiner right now and I noticed that the USPTO might let you switch to part-time with full benefits (that you pay more out of pocket for) after you reach GS-11. It requires approval, they have a limited number of slots for it, and the vast majority of the slots for it require a justification like parenting or eldercare.
The minimum number of hours you can work per week is 16 in the program, and you can choose which days you work. To maximize federal holidays off, pick Monday and Thursday as work days. (I think examiners need to work Thursdays from 1-2 pm due to USPTO rules, and you also get Thanksgiving this way.)
You can find some information about the USPTO's part-time program here: http://www.popa.org/about/work-life-balance/part-time-progra...
(The USPTO is also well-known for their work-from-home program.)