Employers are not technically required to give employees vacation time, but most do, for the simple reason that giving employees time off motivates them and serves as a perk of the job. (Employers are, however, required to give employees up to 12 weeks of unpaid leave under the Family Medical Leave Act (FMLA) if they are caring for a sick relative or a baby or under other similar situations.)
In crafting your letter to employees, you want to explain clearly why there is a change in your policy. For example, if you are allowing them more vacation time (or allowing certain employees more vacation time as a reward), you should explain how employees can earn this extra time. If you are taking away vacation time, you need to explain that too. If you are taking away vacation time, you could consider adding vacation time later on if employees meet an organizational goal, and you could make the medicine less bitter by offering them lunch or other rewards on the days they have to work. You should provide notice of changes far in advance so that employees do not have conflicts with the changed work schedule. In general, this letter should come from the human resources department and read something like the following:
To: All Employees
From: HR Department
Date: [fill in the date.]
Please be advised that all employees will be required to work on the Friday before the Labor Day holiday this year. While this day was granted to all employees last year, this year's increased sales volume has required us to work on that day. If we meet our sales goals by November 1, we will grant an additional day off over the winter holiday. Rest assured that we are asking for this additional day only because it is necessary, and all members of the administrative team will also be working on that day and will provide free lunch to employees on that day. If you have any questions or concerns, please contact [fill in the name] in the Human Resources Department at [e-mail address or phone number].
Matters like "changes in number of days off per calender year", applicable to all the employees, form part of a company's rules, regulations, and policies. Information on such matters needs to be incorporated in company's systems and procedure manual.
Intimation to all employees on such matters can follow one of the two patterns. In first pattern, the the changes are incorporated in the basic manuals of the company independent of the intimation to individual. In addition intimation is given to individual employees of such changes giving appropriate reference to the part of the manual affected by such change. In the second pattern them the intimation to individual employees it self constitutes the document effecting the change in manual, which is then attached by employees to copies of manual. Maintained by them.
With the use of on-line manuals in many companies, it is perhaps not necessary for individuals to maintain individual copies of the manuals, and we can assume that the email communication referred in the question is of the second type - that is, its primary purpose is to inform rather than constitute a formal document covering all detailed contractual aspects between the company and its employees. In a case like this the email could have a format similar to one given below.
Sample format of an email intimating change in changes in leave rule.
From: Human Resources Department
To: All employees
Subject: Leave Entitlements
Please not that with effect from ................., the leave entitlements of employees has been changed from .... days per year of service to ....days. For complete details of the leave rules please refer to Section .... of the company's Employment Rules Manual.