بسم الله الرحمن الرحیم
From the 1st of Zilqada to 10th of Zilhijja are the days of Arbaeen-e-Kalimiyyah. These are the forty days when Nabi Musa (as) went on Mount Tur for Munajat from Allah (swt). Initially it was to be for 30 days, another 10 days were added to make it 40 days as mentioned in Sura A’araaf - Aayat 143. During these days recitation of 'LAA ILAAHA ILLALLAAH ' is mustahab and very much stressed. (Source: Tazkiye Nafs by Ayatullah Misbah Yazdi)
Fasting for 3 Consecutive days Thursday Friday & Saturday has immense rewards in months of Zilqad, Zilhajj, Moharram & Rajab.