Direct answer: In most contexts, you are not expected to say “Happy Veterans Day.” The holiday is about honoring and thanking living veterans for their service, rather than celebrating a festive occasion, so a more appropriate greeting is to express gratitude for their sacrifices. Guidance on common phrasings:

• Best practice: say “Thank you for your service” or “I appreciate your sacrifices.”
• Alternatives: “I’m grateful for your service and dedication,” “Thank you for protecting our country,” or a simple, respectful acknowledgment tailored to the person.
• If addressing a group or public audience: “Thank you to all veterans for your service.”

Notes and nuance:

• The distinction often matters between current service members and veterans. For active-duty service members, some people still opt for respectful, non-celebratory language appropriate to their status.
• When writing cards or messages, focus on appreciation and respect rather than celebration. For example: “Thank you for your service and sacrifice. Happy Veterans Day” should be avoided; instead, use “Thank you for your service. Happy Veterans Day” only if the context and recipient would welcome that wording.

If you’d like, share the situation (e.g., a card to a specific person, a public post, or a workplace note), and a tailored message can be crafted.