3 DAY NOTICE TO CURE BREACH (sometimes called PERFORM COVENANT) OR QUIT is used to notify a tenant that he/she has 3 days to do or stop doing something as per the rental agreement or the law. The tenant can comply with the notice or vacate within the 3 days. If the tenant vacates within the 3 days, he/she is NOT relieved from the rent obligation under the lease or rental agreement.
So, the notice must be based on a law and/or the lease agreement. Presumably, the law and/or lease provision must be cited in the notice.
The City of San Diego has a GOOD CAUSE law (Right to Know Ordinance) protecting certain tenants in a residential tenancy of at least TWO years and that good cause must be written in the notice.
You state that you've been there seven years, so this law applies.
To count the days of a notice, you begin on the next day after service as the first day. Weekends and holidays are counted but the last day of a notice to act generally may not land on a weekend or holiday. If it does, and if applicable to that notice, the "last day" can carry over to the next business day. For example, if a 3 day notice to pay rent is served on a Thursday, we count Friday as the first day Sunday is the third day. Since the last day of this type of notice cannot be a Sunday, the "third day" is then Monday (giving the tenant four days instead of three) to pay the rent. If that Monday was a legal holiday, then Tuesday would then be the "third" day (giving the tenant five days instead of three) to pay the rent.
B. SERVE THE THREE-DAY NOTICE
1. After the Three-Day Notice has been filled out and signed, you must
serve it on the tenant. The Three-Day Notice may be served either:
(a) By delivering a copy to the tenant personally, or
(b) If the tenant is absent from is place of residence and from his
usual place of business, by leaving a copy with some person over the age of eighteen (18) years
at either place, AND mailing a copy of the Three-Day Notice addressed to the tenant, postage
prepaid, first-class mail, to his place of residence; or
(c) If such place of residence and business cannot be ascertained
and a person over the age of eighteen cannot be found at the tenant’s residence, then by affixing
a copy of the Three-Day Notice in a conspicuous place on the property, AND mailing a copy of
the Three Day Notice to the tenant at his residence.
So if the notice was just left at your apartment, and was not delivered personally or mailed, then the service is invalid.