Is it fine:
1.A special stamp has been issued for the group's twenty-years’ activity;
2. the show celedrated the group's twenty years of activity
Last edited by minnie2007; 01-Jun-2007 at 18:10.
The word 'activity' sounds a bit out of place. I'd expect something like 'A special stamp has been issued to celebrate twenty years of the group's work' or even '... to celebrate the group's twentieth birthday' or just '...to celebrate twenty years of [whoever] [in the field of [whatever]]',