Our tinyhouse deadline: just when is the start of spring, anyway?

Ok I know I said we’d be in our tinyhouse by Spring. And we’re so achingly, so un-utterably, so plainly not in. Yet. But just when does spring start? Is it a date thing? An equinox thing? Can i find an adequate caveat? You know those wattle-and-daub walls of ours? Great idea. No really. Only…