If that doesn't make sense, consider the following examples, both of which come from my thesis:
1. "...coincided with the 75th anniversary of the foundation of the Republic..."
2. "...a 75th-anniversary display which featured..."
Is the second example correct, or would you call it a "75th anniversary display", without the hyphen? I've believed the hyphen to be correct, but a friend who proofread my thesis questioned the usage, and I'd prefer not to be making a mistake.