The default list template(s) attempt to assign the month using strftime %m to a
variable $month_number.  ($event.event_date_start|date_format:"%m")

Then in the display this month_number is used as an index to the month_names
array like this:

$month_names[$month_number]

The problem is that %m yields a two-digit month number, such as 01 for January. 
The month_number array uses 0 for January.

01 is interpreted by Smarty as a string, and $month_names['01'] is null, so no
month name is displayed.

There are several ways to fix this problem.  Below is my way.

Solution:

Step 1. Delete these two lines that assign the month number:
{assign var=month_number value=$event.event_date_start|date_format:"%m"}
{assign var=end_month_number value=$event.event_date_end|date_format:"%m"}

Step 2. Find and Replace Start Month

Replace This: {$month_names[$month_number]}
With This: {$event.event_date_start|date_format:"%B"}

Step 3. Find and Replace End Month

Replace This: {$month_names[$end_month_number]}
With This: {$event.event_date_end|date_format:"%B"}