Simply replace the unfilled areas such as "" and "[date]" with your information.
[Date] will mark your fifth anniversary as a member of [name of firm]. We would like to take this opportunity to thank you for these past five years of fine work and company loyalty.
We know that the growth and success of our company is largely dependent on having strong and capable staff members, such as yourself, and recognize the contribution you make in helping us maintain the position we enjoy in the industry.
We are hoping that you will remain with us for many years to come and would like to offer our congratulations on this anniversary.