The first day of the islamic week is Yawm-ul-Ahad, that is Sunday.
Hazrat Anas Bin Malik (r.a) reports that the sahabas (r.a) asked the holy Prophet about yawm-ul-Ahad (Sunday), the holy Prophet answered: "It is a day to plant and begin a construction ". And when the sahabas (r.a) asked for the reason, then the holy Prophet answered: "Because it is verily on that day, that Allah ta'ala began the creation of the Earth".
For this reason, to sow, to plant, to begin a construction or a significant work on that particular day, attract the blessings of Allah ta'ala.
When the Christians of that time, said concerning Yawm-ul-Ahad, that it was so-called one day which belongs to them, Allah ta'ala contradicted by sending a revelation (wahi) to the holy Prophet to declare that this day bears the name of Allah, "Ahad". Indeed, it is one of the attributes of Allah. Thus "Qul huwallaahu ahad " which means "Say, he Allah is ONE ". Therefore, the name Yawm-ul-Ahad comes from that attribute of Allah. Thus, Yawm-ul-Ahad means "First Day ".
Hazrat Ali (r.a) said that on that day, it is necessary to make namaz nafils. And also between the sunnat and faraz of the daily 5 namaz, make the tasbih: "Laa ilaaha illaahu wahdahu laa shariika lah " (There is no divinity except Allah, he is unique and has no associate). Hazrat Ali (r.a) said that by the barkat (blessing) of this tasbih, our faith (imaan) will be so firm that it will never waver and also will never, even a little, turn towards idolatry (shirk). Our heart will be never leaning towards another religion but Islam, and when Allah will remove us from this world, our faith will remain firm on Islam and our heart will leave with the formula "Ash-hadu anl-laa ilaaha illallaahu wahdahu laa shariika lahu, wa ash-hadu anna sayyidanaa Muhammadann `abduhu wa rasouluh" (I testify that there is no god but Allah, he is unique and has no partner and I testify that our leader Muhammad is his servant and messenger).
Dear brothers and sisters, if you do not have time to make enormously this tasbih, therefore you can read the 3rd kalimah (kalima-shahaadat) once after each namaz during Yawm-ul-Ahad (Sunday), as advised by a very great wali-Allah, Hazrat Imaam Al-Ghazzaali (r.a).