Green Christmas, white Easter.

The proverb predicts that when Christmas arrives without snow, snow will come at Easter instead. Its paired colors turn a contrast between the holidays into a weather forecast.

It can also express the thought that unusually mild weather may be followed by cold weather later in the season. The prediction is proverbial, not a dependable rule.

Use it when Christmas weather is mild or the ground is free of snow, especially to suggest that wintry weather may arrive around Easter. It can be said lightly rather than as a serious forecast.