Simply replace the unfilled areas such as "__" and "_[date]_" with your information.
To All Employees,
The management of _[company or department]_ would like to invite everyone to come out and enjoy some food and fun on us. You have all worked so hard this past _[length of time]_, and we would like to show how much we appreciate all you do for the company and the customer.
The Employee Luncheon will be held at _[time]_ on _[date]_. The Luncheon will be located _[location]_. We will serve _[including a summary of the menu]_, and there will be entertainment in the form of _[list entertainment]_.
Everyone is encourages to attend and enjoy the food and company.