When something betides you, it happens to you. Betide is a literary way of saying “happen,” like in this quote from Jane Austin’s Persuasion, “Woe betide him, and her too, when it comes to things of consequence….”
This is an old-fashioned word for things happening, especially uncertain or negative things. This word often appears with woe as in "Woe betide all who date my sister!" Which means, basically, something bad will happen to you if you date her. If you say, "What will the new day betide?" it means, "What will happen tomorrow?" Betide means about the same thing as bechance and befall. We all want good things to betide us.