上街 is a separable verb (离合词) literally meaning 'to go onto the street', commonly used to mean 'to go shopping' or 'to go out'. When you add a time duration, number of times, or other complements, you must split it: put the extra element between 上 and 街, like 上了一次街 ('went to the street once') or 上了半天街 ('spent half a day street-shopping'). The full form is used when no complement is added.