last month本题询问何时进行的顾客调查。根据文章第一句话"Now I’m going to tell you the result of the customer survey we conducted last month"可知调查是上个月进行的

Now I’m going to tell you the result of the customer survey we conducted last month. We got high marks from our customers about the quality of our products．They felt our products were quite reliable and carried a lot of the features they expected in electronics．They especially liked the hi-tech mobile phone，which combines a mobile phone with a camera and mp3 player．However，customers were not satisfied with our customer service．They complained in the survey that it took too long to settle their complaints．They also complained that our service staff were not always friendly. By contrast，they wrote that many of our competitors have excellent customer service．This is the urgent task we have to deal with now．That’s why I’m ordering the customer department to come up with a plan within one week and I also asked them to explain how they intend to improve our customer service．