Short answer. Upon your father's death. Article 945 of the Civil Code lets a legatee bequeathed a periodical pension petition the court for the first installment as soon as the testator dies, with later installments due at the beginning of each following period.
What the law says
If a periodical pension, or a certain annual, monthly, or weekly amount is bequeathed, the legatee may petition the court for the first installment upon the death of the testator, and for the following ones which shall be due at the beginning of each period; such payment shall not be returned, even though the legatee should die before the expiration of the period which has commenced.
Civil Code, Article 945 — Periodical Pensions. Read the full provision →
When the right to the first installment arises
Article 945 addresses bequests of a periodical pension, or a certain annual, monthly, or weekly amount. For this kind of legacy, the article says the legatee may petition the court for the first installment upon the death of the testator. This means your entitlement to the first payment is tied directly to your father's death as the triggering event, rather than to some later date tied to the settlement of the estate more generally.
How later installments work
Article 945 also addresses the payments that follow the first one: they are due at the beginning of each period, meaning each subsequent monthly installment becomes due as each new month begins, following the same periodical rhythm the will established for the pension. This creates an ongoing, recurring right to installments as time passes, rather than a single lump-sum entitlement paid out all at once at the start.
Why installments already due are not returned even if you later die
Article 945 adds a protective rule: such payment shall not be returned, even though the legatee should die before the expiration of the period which has commenced. This means that once an installment has become due and been paid to you, your death before that particular period fully runs its course does not require the payment to be given back to the estate. The installment, once due, is treated as fully yours for that period.
What this means practically for collecting your pension
Because your right to the first installment is tied to your father's death, that is the point at which you may petition the court for it, rather than waiting for the estate to be fully settled or for some other administrative step to occur first. Understanding that each later installment becomes due at the start of its own period helps you track when subsequent payments should follow, consistent with the periodical structure Article 945 describes for this kind of legacy.