In many courts, witnesses receive nominal fees but are not entitled to compensation for missed work. ON the other hand, you cannot be fired for missing work. Unfortunately, if you have been summoned to testify, it is your civic duty to do so.
Legal Disclaimer: You should remember that this answer does not constitute legal advice, and should not be relied on. This is because each state has different laws and each situation is fact specific. That is why a lawyer can't evaluate a legal problem without a consultation and a chance to review every fact and document at issue.
Also, this answer does not create an attorney-client relationship between us.
Your summons is a lawful order of the court requiring you to appear. This is not "an invitation". It is legal process and you can be held accountable for your failure to appear. Court appearance fees are allowable for witnesses. It depends on what jurisdiction you are appearing in as to how you would be paid for your absence, however, it is rarely equal to a regular day's pay.
My comments are not intended to establish an attorney-client relationship, are not confidential, and are not intended to constitute legal advice. Proper legal advice can only be given by an attorney who agrees to represent you, who reviews the facts of your specific case, who does not have a conflict of interest preventing the representation, and who is licensed to practice in the appropriate jurisdiction where the legal issue may be filed or in the state where the law applies.