Important information that should be in an internship offer letter. Use this sample letter to detail a job position to a temporary employee, stating the period of time for which he or she has been hired.The internship offer letter should include the start and end dates of the internship, as well as the number of working hours per week. An internship offer letter is a written document that formally offers an internship to a potential intern. Download a sample template for free. Congratulations on this new opportunity.