My bet would be about 8 months before the last episode of Daredevil.
When Karen is looking through old papers (s2E5), she mentions the date "April 14th" as approximate time when information about Frank's family should be printed.
Karen: Well, these are all the papers from the week that Castle was shot. I've been through them three times and there is not a single mention of Castle, his family... not even John Doe that matches. Just nothing
Mitchell : Well you know, people are shot every day. It doesn't always make the paper [...] April 14th... When was the Castle's family murdered?
Karen: Well, I've figured out it must been sometime that week, right?
Mitchell: You know, I cannot remember my kids' birthdays, but violent, soul-sucking events are just seared into my brain.
In last episode Mitchell gives Karen a Christmas gift, so we can guess that the action of the series happen around the end of December
Unfortunately I don't know exactly how much time passes between first and last episode of Daredevil, but as @Kevin Workman pointed it seems that there is quite hot outside, which would put it at summer/early autumn.