إنّ أميرَ المؤمنينَ صلواتُ اللَّهِ علَيه لمّا حَضرَهُ الّذي حَضرَهُ قالَ لابنهِ الحسنِ : اُدْنُ منّي حتّى‏ اُسِرَّ إليكَ ما أسرَّ رسولُ اللَّهِ صلى اللَّه عليه وآله إليَّ ، وأئتَمِنَكَ على‏ ما ائتَمنَني علَيهِ ، فَفعَلَ.

When death came to the Commander of the Faithful [Imam Ali] (PBUH), he said to his son Hasan, 'Come closer to me so that I may tell you a secret which the Prophet of Allah (SAWA) told me, and that I may entrust to you what he entrusted to me. And so he did.

al-Kafi, v. 1, p. 298, no. 2

According to the lunar calendar, the 28th of Safar coinciding with Sep. 2 of this year marks the martyrdom anniversary of Imam Hasan al-Mujtaba (PBUH).