From digging around more I think finer scheduling is not possible directly through the filer.
I also found that the manual tells something wrong about snapshots exception on sunday.
"man snap" says "Daily snapshots are created at 24:00 of each day except Sunday, and weekly snapshots are created at 24:00 on Sunday."
I have configured 4 daily snapshots no weekly and no hourly ( = snap sched 0 4 0) and I very well see a snapshot from 24:00 at sunday.
This is even more awkward as it means that during an inactive weekend 2 valuable snapshots from weekdays are scrapped in favour of almost nothing.
My guess is that the manual wants to point out that if weekly snapshots are defined these will replace the nightly snapshot on sundays.
Thus if weekly AND nightly snapshots are configured there will be no nightly snapshot on sunday.
So I deciced to make my own snapshot scheduling.
I added a public key to vol0/etc/sshd/root/.ssh/authorized_keys and will use a script which I schedule from a linux server using cron.
This way I have full control over snapshot scheduling and the flexibility I want.
The sacrifice is that snapshotting now depends on an external host