Figure 40K tickets at an average of $300 = $12 million (conservative)
Figure each fan spends $50 on food or merch = $2 million (conservative)
Figure the average for travel is $200 for each fan = $8 million (conservative)
For $22 million -- probably closer to $30 million -- being spent, they're not going to cancel the concert because of heat.
Also remember, once the sun goes down, the temperature will drop as Boston is right on the water.