Mark Sandusky has a good article on that topic, here is one excerpt:
Time your busks wisely! Profits can vary widely from day to day, hour to hour. Our low for a Friday night was $98 for two hours of performance. Our high for two hours of performance on a Monday afternoon was $3. This was also our low, because we never busked on another Monday afternoon. We made the most money in between 5pm and 10pm, on evenings before weekends or holidays. Our understanding is that money drops best when people are feeling tipsy, but before they’re actually drunk.
The piece serves up other points of interest.