Wisely, you call the attorney who issued the subpoena. You mention to the secretary that you have never been called as a witness previously. The secretary schedules a time for you to speak with the attorney.
You need to speak to the attorney to figure out exactly what the subpoena requires you to do. For example a subpoena duces tecum (latin for “under penalty to bring with you”) may require only the production of documents -- you may not testify at all. If you are going to testify, the attorney may review the questions he or she intends to ask. The attorney may, or may not, be aware of specific questions the opposing attorney is likely to ask you on cross examination.
When you called the Center for Miracles (you did call the Center for Miracles, didn’t you) you learned about standby. Very few trials happen on the day they are originally scheduled, and you can waste a lot of time showing up for all of them. Many attorneys will allow you to be on “standby” rather than appear at court. You will be required to give a phone number at which you can be reached on the day of trial. If you are called on the day of trial, you would need to show up within 20-30 minutes. When the secretary gives you permission to be on standby you write his or her name and the date on your subpoena.
The day before trial arrives. What do you want to do?
Pull the chart and take it with you. You can always review the chart on the stand.