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.