* As a formal greeting: "Seasons Greetings" would be capitalized like any other greeting, similar to "Happy Holidays".
* In a casual context: "Seasons greetings" would be lowercased.
Here's a breakdown:
Formal:
* On a holiday card: "Seasons Greetings"
* In a business email: "Seasons Greetings to all our valued customers"
Casual:
* In a text message: "Hope you have happy seasons greetings!"
* In a conversation: "I just wanted to wish you happy seasons greetings."
Ultimately, the best way to know is to consider the tone and context of your writing.