The twenty days is from the date you received it.  You will need to file an answer to the complaint.  The complaint is the lawsuit attached to the summons.  The answer denies the allegations in the complaint and needs to be filed with the court and served by mail on the opposing party.  If you have an attorney, the attorney can handle that for you.  If you don't have an attorney, look for answer to complaint in the index of Pleading and Practice at the law library.  This will give you the general format for an answer to a complaint.  At the end of the answer is the verification which you sign and date under penalty of perjury.  The verification attests to the veracity of your statements in the answer.  File the answer to the complaint with verification and an attached proof of service with the court and mail a copy to the opposing party.  The proof of service verifies the date of mailing your documents to the opposing party.  You can either use a court form proof of service or you can write your own.  If you write your own proof of service, it just says that you are at least eighteen and the attached documents were sent via first class mail unless stated otherwise to ____________ (name and address of opposing party or opposing party's attorney) on _______ (date).  You sign and date at the bottom.  The date you sign should be the same as the date of mailing and the same date you file your documents with the court.If you don't file a timely answer to the complaint with the court and serve it on the opposing party within the twenty day deadline set forth in the summons, the opposing party can file a default and obtain a judgement against you.  If that happens, you will need to file a motion to set aside the default.  If the court grants your motion to set aside the default, the case is then back on track and litigation will continue.