Some people say that it is Haraam to set aside important days, such as Mondays and Thursdays, Giyaarwi Shareef (11th of the Islamic month), Chatti Shareef (6th of the Islamic month),etc. to perform some virtuous act. The Ahle Sunnah wal Jama'at believes that it is not Haraam to set aside special days for Islamic acts such as Moulood-un-Nabi, Giyaarwi Shareef, Esaale Sawaab, Urs Shareef, etc. Allah Ta'ala says in the Holy Quran: "And remind them of the days of Allah" (Part 13, Ruku 13)

Rasoolullah (sallal laahu alaihi wasallam) himself set aside specific days, namely:-

1. He used to keep fast on Mondays and Thursdays. (Mishkaat Shareef; Abu Dawud Shareef)
2. The Holy Prophet (sallal laahu alaihi wasallam) would go every Saturday to Musjid-e Quba sometimes travelling on horseback and sometimes walking. He would perform two Raka'ats of Nafil Salaah there. (Sahih Bukhari; Muslim Shareef)
3. Rasoolullah (sallal laahu alaihi wasallam) went into Ghazwa-e-Tabook on a Thursday and he preferred to journey on a Thursday. (Sahih Bukhari; Mishkaat)
4. The Holy Prophet (sallal laahu alaihi wasallam) told Sayyiduna Abu Zar Ghaffari (radi Allahu anhu) that if he wished to fast, then he should fast for three days in a month, namely the 13th, 14th and 15th of every Islamic month. (Mishkaat; Nisaa'i Shareef)
5. It has been related by Sayyiduna Anas (radi Allahu anhu) that the Holy Prophet (sallal laahu alaihi wasallam) used to visit the graves of the Martyrs of Uhud in the beginning of every year. Thereafter, the orthodox Khulafa too kept on doing so. 6. It has been evident from this that the Holy Prophet (sallal laahu alaihi wasallam) used to approach the Martyrs on a particular day. (Tafseer Durr-Manthoor, Tafseer Kabeer)

Then, to set aside specific days to perform certain Islamic actions are totally permissible.