Sayyidunā Milhān Al-Qaysī radhiyallāhu 'anhu reports, "Rasūlullāh sallallāhu 'alayhi wasallam would encourage us to keep the fasts of Bīd i.e. the thirteenth, fourteenth and fifteenth." He further narrates that Rasūlullāh sallallāhu 'alayhi wasallam mentioned, "It is equivalent to fasting every day." (Abū Dāwūd)
This is because Allāh ta'ālā through His infinite grace has promised reward ten-fold upon each action. Therefore, fasting three days is actually equivalent to fasting thirty days i.e. an entire month. Hence, the reward of practising upon this every month will be equal to the reward of one who fasts every single day of his life.
Rasūlullāh sallallāhu 'alayhi wasallam himself punctually fasted on these days and also encouraged his followers to do the same. According to the weightiest opinion, the 13th, 14th and 15th of every lunar month have been titled Ayyām-ul-Bīd, the days of Bīd.
All readers are urged to make the most of this opportunity and practise upon this sunnah of Rasūlullāh sallallāhu 'alayhi wasallam.