It can be ominous. With the barrier between the two worlds being thin, all manner of spirits may slip through and linger through the cold months.

Samhain is that. The celebration of the coming of winter, the time of harvest, the loss of the sun. It's the time when the door between the living, and the otherworld, is thin. In order to appease the spirits that visit, whether they be the dead or otherwise, offerings are to be made, and celebrations are to be had. Lest those very same spirits cause problems during winter.