The Wall Street star turns 65, while his wife will reach the landmark age of 40 later this week.
But instead of throwing a lavish showbiz bash, the pair is determined to keep it a family affair, and will toast the birthdays with a meal in New York - with just their family and close friends in attendance.
A source tells U.K. newspaper the Daily Record, "Catherine doesn't want to have a big bash, she'd rather just have an intimate gathering with her closest friends and family.
"The last thing on her mind is having a party that the good and great will come to. Catherine loves nothing more than to spend time with her family. She's not big into showbiz bashes, so it will be a quiet affair."
Kirk Douglas, 93, and Catherine's parents Dai and Pat will help the happy couple celebrate the big day with the meal in Manhattan.