...And so are the Days

{And so are the days (good and not so good), that We give to men by turns, that Allah may Know those who believe

Ibn Abi Haatim's comments:

Muhammad ibnul 'Abbaas narrated to us that Muhammad ibn Ishaq said:

{And so are the days, that We give to men by turns... He said: "We give them to men with trails so that we may test them." His - the Glorified's statement: { that Allah may Know those who believe.

My Father narrated to me that Ahmad ibn 'Abdur Rahmaan narrated to him…on the authority of Rabee' ibn Anas:

"Allah gave victory to His Prophet and his companions over the polytheists on the day of Badr, and He gave victory to their enemies on the Day of Uhud. A non-beliver can be given victory over a believer, and a believer can be tested with a non-believer. So that those who obey him may be distinguished from those who don't, and those who are truthful from those who are not."

Tafseer Ibn Abi Haatim: 3/773

As Sa'dee comments:

{And so are the days, that We give to men by turns...

The wisdom behind this is that in this world, Allah gives the believer as well as the non-believer, the righteous as well as the transgressor. So Allah shifts the days amongst people, one day for this group, and another day for the other. This is the worldly life which will come to an end, in contrast to the afterlife, for indeed it is specifically for those who believe.

{ that Allah may Know those who believe

It is also from wisdom that Allah tests His servants with defeat and trails, so that the believer is distinguished from the non-believer. If the believers were to continually be victorious, people who do not really want Islam would embraced it. So if there are some trials in some cases, the true believer who really wants to be Muslim would become clear, in times of ease and hardship, he would be distinguished from other than him.

Tafseer as Sa'dee: 1/149