Sayyidul Istighfar (Master of seeking forgiveness)

When. Morning and evening. Its name — sayyid al-istighfār, the master of seeking forgiveness — is the Prophet's ﷺ own description of it, and the hadith attaches a promise to saying it with certainty in the morning or the evening.

Why it is called the master of them. Other forms of istighfār ask for forgiveness. This one argues for it. Read the order of the sentences: it establishes who Allah is, then who the speaker is, then what the speaker has done, and only then asks. Nothing is requested until the relationship has been stated, and the request when it comes rests on the last line — none forgives sins but You — which is not flattery but the reason the request has nowhere else to go.

The words that carry it. Anta Rabbī — "You are my Lord." The whole supplication is built on this being said first. ʿAlā ʿahdika wa waʿdika mā-staṭaʿtu — "upon Your covenant and Your promise as much as I am able." That last clause is the honest one. It does not claim to have kept the covenant; it claims to be holding to it within a capacity that is limited, and admits the limit in the same breath. Abūʾu laka bi-niʿmatika ʿalayya wa abūʾu bi-dhanbī — the same verb twice, "I acknowledge / I return bearing". Blessing and sin are carried in with the same hands. Naming both is the point: a confession that mentions only the sin is half the account.

Practical. It is long, and it is meant to be. Learning it in parts, a line at a time, is how most people come to it. There is no fixed count — once in the morning and once in the evening is what the hadith describes.