It is a small pedestrian street located between Pavlou Mela and Iktinou streets. It is filled with small coffee-bars full of people from the morning till late at night.
It is a favourite city spot for many reasons. First of all, it has an ideal location where it is cool to sit outside in the summer but also not so windy and cold in the winter. Secondly, it is a car free zone. Thirdly, you can find people of all ages and styles, although the majority is young students. Last, and maybe most important, the bars play music only on the inside, so sitting outside is cool and quiet and you can actually hear your friends talking, in contrast to the popular nightlife area of Valaoritou where when sitting outside you just listen to music from all the bars as one loud noise.
So I suggest you drop by Zefxidos street anytime of the day, and enjoy the last summer days of September and October. My favourite time to go there is for morning weekend coffee, or early weekday evening drinks.