I'm no expert in necromancy, as I don't really use it, but from what I can recall it's a form of divination that can allow one to communicate with spirits. Nowadays media portrays it as necromancers doing a dance around wierd colored fire in skimpy grass skirts, raising the dead. However, it doesn't work that way in real life. In real life you conjure up souls and do 20 questions with them (or as many as you can ask within the time that you are able to maintain the concentration required for the ritual.)
From my limited understanding on the topic, if you attempt necromancy on Halloween and Samhain, the veil between worlds is thinner and it should be easier. I have also read that strong magic can be performed by using the ashes of a strong priestess/priest and by attempting magic in a cemetery where a witch is buried, I consider these to be form of necromancy despite the fact that you are not communicating with them because you are using their power for your own magic
Huggles and blessings.