A popular 50th anniversary card wish is "To celebrate your golden wedding anniversary we send you many greetings and wish you many more years together in harmony." There are many other options, depending on the relation of the person writing to the couple celebrating their anniversary.
What you should write on a 50th anniversary card depends on whether you plan to write it for a spouse, parent, friend, colleague or acquaintance and whether the couple celebrating the marriage are religious. For a spouse, a suitable wish is "You lean on me, and I lean on you. What a blessing it is to have someone to lean on for 50 years!" A wish that works well for friends and family members of the couple is "50 years means countless opportunities to give up, but you never did. Congratulations for your perseverance." For a religious couple, one can write "I don't know how anyone could stay together as long as you have, unless God helped you do it. Congratulations!"