Part 1: The dates
This something really interesting I found out, and it's regarding the dates.
Freakishly enough, the calender dates for June of 2013 is identical to that of March 2014. So if June 13, 2013 is on a Thursday, then March 13, 2014 is also on a Thursday. The killer seemed to have skipped 2015-2016 because 2016 was a leap year.
Another point related to calenders
The lunar cycle also oddly relates to the dates, except in alternating cycles. Here's what I mean. On June 8th, 2013, it was a new moon. On March 8th, 2014 however, it was the end of the first quarter, meaning it was one moon phase ahead. However, the moon phases in June of 2015 and March of 2016 don't align. The same goes for June of 2016 and March of 2017, where none of the dates align. However, they fall into the alternating cycle again in June 2017, where the first of that month was the 1st quarter of the moon phase. And (you guessed it!) March 1st, 2018 it was a full moon (or second quarter of the moon phase).
In conclusion
The murder has to happen this month. It has to be after the 14th, though. I think the dates/how the times of the murder is important to the location. The murderer gives the clues in March, because the cycle restarts in June. So if we could connect the dates to the location... that might give us our lead we need.
Other thoughts, though this part might be irrelevant
Maybe the murderer is like some weird werewolf that attacks only during like June and March. I'm not sure about anything more significant, besides the fact that Daylight Savings start in March and Summer Solstice happens in June.
EDIT:
I just realized Spring Equinox happens on March. March 20th, to be exact. Summer Soltice happens on June 21st. Another significant fact: This year, Spring Equinox happens on the same day as the full moon... relevance?