From ‘Aaishah radi Allaahu ‘anhaa who said:
There were some fasts from Ramadaan which I had to make up, so I was not able to make them until Sha’baan.
The Shaykh Muhammad ibn Saalih al ‘Uthaymeen rahimahullaah commented:
‘Aaishah radi Allaahu ‘anhaa informs that there were some fasts from Ramadaan which were upon her; she had refrained from fasting (during those days) due to an excuse, by way of her menstrual periods or other than that.
So she delayed them until Sha’baan, then she fasted them before the next Ramadaan.
And she clarified her excuse for that – that she was unable to make them up easily and conveniently until Sha’baan, when it became essential to make them up.
 The month before Ramadaan the following year
(taken from Sh Uthaymeen’s explanation of ‘Umdat-ul-Ahkaam
translated by Nasser ibn Najam)