When you receive a subpoena, you literally have a command with the force of the court to appear as stated. This is why in most places only an attorney may issue a subpoena. The question often arises whether the person receiving the subpoena must appear.
First of all, look at the bottom of the subpoena, there should be a name and telephone number identifying the attorney who issued the subpoena. Call the attorney. Ask whether this is really going to go forward. Or, if work is a major conflict, ask whether the appearance can somehow be rescheduled.
If the subpoena is for a deposition, these can often be rescheduled. If it is a trial, these do not always go as planned and your appearance can be scheduled so that you are not sitting around all day doing nothing waiting. The best case scenario, which may not always fit your circumstances, is if you are a relatively short distance from the court, you can be put on cell phone standby.
Employers needs to understand that if an employee has been subpoenaed to appear in court, the failure to go can result in a finding of contempt of court. In some circumstances this can lead to imprisonment. It is a public policy violation for an employer to not recognize an employee's duty to go to court when subpoenaed.