Yes, people living in the southern hemisphere would be experiencing spring.
It would not make sense to celebrate Harvest at Spring planting time. 
A couple of good books to check out: Witchcraft, Theory and Practice
When I See The Wild God
Both are written by Ly de Angeles, who is a witch living in Australia. Ly also has a Facebook page. She has also reissued one of her older books: Genesis. (I think the rest of the title is A Legend of Future Past).
Ly also has written some excellent fiction books: The Quickening (recently revised and re-issued), and The Shining Ones. I can highly recommend all of her books!
Even though it is Fall in my part of the world, I wish you happy planting if it is Spring where you live.
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As a bonus, making a contribution will allow you to leave kudos for another DU member, and at the end of the week we'll recognize the DUers who you think make this community great.