Short answer. If the lease is silent on where and when to pay rent, Article 1679 of the Civil Code fills the gap: the place of payment follows Article 1251, and the time follows the custom of the place. The parties' own stipulation, if any, always comes first.
What the law says
If nothing has been stipulated concerning the place and the time for the payment of the lease, the provisions or article 1251 shall be observed as regards the place; and with respect to the time, the custom of the place shall be followed.
Civil Code, Article 1679 — Place and Time of Rent Payment. Read the full provision →
Defaults only fill gaps
A lease can, and usually should, state exactly where and when the rent is to be paid. Article 1679 of the Civil Code steps in only when it does not. It applies If nothing has been stipulated concerning the place and the time for the payment of the lease. So the parties' own agreement always controls first; the article is a gap-filler, not an override. If your contract fixes, say, payment on the fifth of each month at the lessor's office, that clause governs and Article 1679 never comes into play. It matters only when the lease is silent on one or both of these points.
Where rent is paid: Article 1251
For the place, Article 1679 says the provisions or article 1251 shall be observed as regards the place, pointing to the general rule on where obligations are paid. Article 1251 works in tiers. Payment shall be made in the place designated in the obligation. If none is designated and the obligation is to deliver a determinate thing, payment is made where that thing was when the obligation arose. In any other case the place of payment shall be the domicile of the debtor. For rent, which is money, this usually means the last tier: absent agreement, the tenant's obligation to pay is, in principle, located at the debtor's, that is, the lessee's, domicile.
When rent is paid: the custom of the place
For the time, Article 1679 takes a different approach. It says that with respect to the time, the custom of the place shall be followed. Rather than pointing to another article, the law defers to local practice. So if, in the locality, house or apartment rent is customarily paid at the start of each month, that custom sets the time when the lease itself is silent. This recognizes that rental practices vary from place to place and that a fair default is the one the community already follows. The custom effectively supplies the due date the parties neglected to write down.
What this means in practice
For a tenant or landlord caught without a clear clause, the takeaways are practical. On timing, look to what is customary in your locality; that custom, not the landlord's sudden demand, fixes when rent falls due. On place, the general rule leans toward the debtor's domicile unless the lease designated somewhere else. Note as well that Article 1251 adds a fairness rule: if the debtor changes domicile in bad faith or after incurring delay, he bears the extra expenses. The simplest protection, of course, is to spell out the place and time in the lease so these defaults never have to be invoked.