[Highlander II: so, is UPN skipping 2 weeks and not airing Buffy at all? or do we get more reruns?]
Actually, the info come from two sources.
One that knows when the next new episodes are "scheduled" to appear, but this far out, that schedule could change.
And regular TV schedule sources that typically publish two weeks in advance.
I doubt if UPN is going to bump Buffy for anything else they've got - after all, what could get better ratings on that network?
But UPN doesn't officially announce what episodes it will play longer than two weeks out. With that in mind, the episode announced for April 9 is #111 Gone (6.11).
Hopefully they will work in a repeat of the Musical while they have the chance.