The Messenger of Allah ﷺ asked a man: Did you fast the last day of Sha'ban ? He replied: No. He said: If you did not observe a fast, you must fast for a day. One of the two narrators said: For two days.

أَنَّ رَسُولَ اللَّهِ ﷺ قَالَ لِرَجُلٍ هَلْ صُمْتَ مِنْ سَرَرِ شَعْبَانَ شَيْئًا قَالَ لاَ قَالَ فَإِذَا أَفْطَرْتَ فَصُمْ يَوْمًا وَقَالَ أَحَدُهُمَا يَوْمَيْنِ