Amy!
Sorry to ask but Kate Smithers (from your work) found out about it being your birthday and would really like to see you so I was wondering if it would be okay for me to bring her along to your open house tomorrow? She would have asked you herself but I don't think she has any of your contact details but apparently you two got on really well at work and she'd really like to see you :)
Let me know if that's okay and don't worry if it's not.
Looking forward to seeing you,
xxxx