Fasting al-Ayyaam al-Beed( White days) is done according to the lunar calendar, not the solar
The virtues of fasting are immense, and a number of saheeh ahaadeeth state this, such as the following:
Fasting is something which is just for Allaah, and Allaah will give multiple rewards without measure for it. The hadeeth says: “[Allaah says:] Fasting is Mine and it is I Who give reward for it.” (Al-Bukhaari, Fath, no. 1904)
“There is nothing equivalent to fasting.” (Al-Nisaa’i, 4/165; see also Saheeh al-Targheeb, 1/413)
“The prayer (du’aa’) of the fasting person will not be refused.” (Reported by al-Bayhaqi, 3/345; see also al-Silsilah al-Saheehah, 1797)
“The one who fasts has two joys: the joy of breaking his fast and, when he meets his Lord, the joy of having fasted.” (Reported by Muslim, 2/807)
Fasting will be an intercession for the slave on the Day of Resurrection, when it will say: “O Lord, I stopped him from eating and drinking during the day, so let me intercede for him.” (Reported by Ahmad, 2/174; al-Bayhaqi classed its isnaad as hasan in al-Mujtama’, 3/181; see also Saheeh al-Targheeb, 1/411).
“The smell (coming from the mouth of) the fasting person is better in the sight of Allaah than the scent of musk.” (Muslim, 2/808).
“Fasting is a protection and a strong fortress against the Fire.” (Reported by Ahmad, 2/402; see also Saheeh al-Targheeb, 1/411 and Saheeh al-Jaami’, 3880)
“Whoever fasts one day for the sake of Allaah, Allaah will keep his face seventy years’ distance away from the Fire because of it.” (Reported by Muslim, 2/808)
“Whoever fasts a day seeking the pleasure of Allaah and then dies [i.e. that is the last thing he does] will enter Paradise.” (Reported by Ahmad, 5/391; see alsoSaheeh al-Targheeb, 1/412)
“In Paradise there is a gate called al-Rayyaan, through which those who fast will enter, and no-one else will enter through it. After they have entered, it will be locked and no-one else will enter it.” (Al-Bukhaari, Fath, no. 1797).
Voluntary fasts will make up for any shortfall in obligatory fasts. Examples of voluntary fasts include ‘Aashooraa’, the Day of ‘Arafaah, al-Ayaam al-Beed (the “White Days”), Mondays and Thursdays, six days of Shawwaal, and increased fasting during the months of Muharram and Sha’baan.
What is meant by al-Ayaam al-Beed is the 13th, 14th, and 15th of each lunar month, because Allaah says (interpretation of the meaning): “They ask you about the new moons. Say: These are signs to mark fixed periods of time for mankind and for the pilgrimage…” [al-Baqarah 2:189]. So the timing of certain acts of worship and the ‘iddah (waiting period for women who have been divorced or widowed, etc.) are worked out according to the lunar months of the Hijri or Islamic calendar, not the solar months of the Gregorian or Western calendar. The fasts which you have observed according to the solar calendar most likely do not correspond with the lunar calendar, but in any case you will be rewarded, in sha Allaah, for the days that you have fasted, because you have fasted them voluntarily for the sake of Allaah. But if you wish to earn the reward that is specifically for fasting Ayaam al-Beed, which the Prophet (peace and blessings of Allaah be upon him) urged us to do, you should find out the timings according to the lunar calendar and fast according to it.
We ask Allaah to increase His blessings upon you, to help you adhere to His religion, to give you the strength to do that which will please Him and to increase your reward, for He is the All-Hearing Who answers prayers. Do not forget all Muslims in your prayers whilst you are fasting. May Allaah bless our Prophet Muhammad.