Is it permissible for someone who is not accustomed to fasting the White Days or Mondays and Thursdays to fast them in one month but not another?

Continuity in performing good deeds of supererogatory nature is better than performing them sometimes and abandoning them sometimes, because the Messenger of Allah, peace and blessings be upon him, said: "And the most beloved deeds to Allah are those that are most continuous, even if they are few." It is recommended for every Muslim who is able to fast to fast these days and to be consistent in that. However, if one leaves fasting them, there is no sin upon him, because they are supererogatory acts, and consistency in them is only recommended.