In different month for each year, usually Jan or Feb.
There is a rule. The Spring Festival means it is the beginning of the new year and the Spring. When is the beginning of Spring? Usually 4th February, also a solar term in China, called 立春 (Beginning of Spring). So it is reasonable to let the Spring Festival around 2.4.
This is based on Chinese Luna Calendric System, Which is actually a mix of solar system and lunar system, and is very scientific.
There are 24 solar terms, 6 for each season. There is also a poem about them.
春雨惊春清谷天 — 立春，雨水，惊蛰，春分，清明，谷雨
夏满芒夏暑相连 — 立夏，小满，芒种，夏至，小暑，大暑
秋处露秋寒霜降 — 立秋，处暑，白露，秋分，寒露，霜降
冬雪雪冬小大寒 — 立冬，小雪，大雪，冬至，小寒，大寒
The first one means the beggining of each season. The fourth ones are important: Spring equinox, Summer Solstice, Autumnai equinox, Winter Solstice. They are coincident exactly with Four seasons, and farmers arrange their activities according to them.