ahsanallahu ilaykum I didnt fast WhiteDays for past 4months because summer days were long. Now days are shorter & soon the fast will be 9 hours. Is it permissible to fast like Dawud عَلَيْهِ ٱلسَّلَامُ for just one month in winter? Or is it insincere to take up this worship only when it is easy?

Ustadh AbdulAziz al-Haqqan

Yes, it is allowed, and this is not insincerity.