The fourth day of the Islamic week is Yawm-ul-arba'a, that is, Wednesday.
Hazrat Ali (r.a), Hazrat Jaabir (r.a), Hazrat Bibi Aïcha Siddiqa (r.a) said that Yawm-ul-arba'a is harmful day. But however it carries bad luck only for the unbelievers (kaafirs), but is blessed (mubaarak) for the believers (mu'minn). In answer to the questions of the sahabas (r.a), the holy Prophet explained that it is on that day that the army of Pharaoh was drowned in the river Nile and Allah sent his punishments (`azaabs) on the unbelievers of the past Wednesdays. As Allah himself specifies it in the holy Quraan: "We sent against them a violent and icy wind, on a harmful and interminable day" [ sourah 54 – verse 19 ]. The harmful day mentioned here is verily a Wednesday.
Just like Qaaroun (of the time of Hazrat Mousa a.s) was absorbed in the ground, Pharaoh and his army were drowned in the river, Namroud (of the time of Hazrat Ibraahîm a.s), received the punishment of the sting of a mosquito which penetrated his brain), the unbelievers of the people of Hazrat Lout a.s (Lot) perished under a rain of rocks, king Shaddaad and the unbelievers of the people of Hazrat Houd (a.s) were carried by a strong gale and many unbelievers among the people of ancient prophets got their punishments on Wednesday.
On the other hand, for believers (mu'minns), Wednesday is a day of Noor, i.e. a day of light. For that reason, from the time sahaabis to our days, Islamic education begins on Wednesdays, because the holy Prophet said that "education is light" and light must begin on the day of light.
Also, Rasoulullaah prevented from cutting the nails and to be made hair cut on Wednesdays because it can generate leprosy. Nabi-e-kareem also advised against making blood transfusion on Wednesdays because it may generate diseases.
Hazrat Ma'az bin Jabal (r.a) reported that the holy Prophet taught to him to make on Wednesdays, 4 raka'ats of namaz nafils in the course of the day (2 by 2 raka'ats), in the first 2 raka'ats, in each raka'at, after sourah al-Faatihah, read 1 time Aayat-ul-kurrsi and 3 times sourah al-Ikhlaass. Separately, for the two last raka'ats, in each raka'at, after sourah al-Faatihah, read 1 time Aayat-ul-kurrsi and 1 time sourah al-Falaq and 1 time the sourah an-Naas. After salaam, make du'as as follows: "Allaahumma rabbanaa aatinaa fid-dunyaa hasanatanw-wa qinaa `azaaban-naarr. Yaa `azîzu Yaa Ghaffaar, Yaa kareemu Yaa Sattaarr". Then make your du'as and by the barkat of this namaz nafil, Allah ta’ala will keep you away from misfortunes and spread light in your life.
Source : http://chezdeen.com/eng/