Hi Eberhard,
Is your report filters data for one year, if it is then use below logic
Create a formula like : (@month)
If Month(datefield) in (5,6,7,8) then 'g1'
Eles
If Month(datefield) in (9,10,11,12) then 'g2'
Eles
If Month(datefield) in (1,2,3,4) then 'g3'
now create a group on @month formula and place your cross tab in Group header / group footer.
Note : please replace datefiled with your table date field (the field which you are dragging in columns)
Thanks,
Sastry