Depends on what you mean by "astronomically". By astronomical observation, the sun passes through the galactic plane every year around the time of the winter solstice (and again at the summer solstice). However, the sun physically passes through the galactic plane as it orbits the milky way, about every 28-33 million years and it is believed to have last happened about 3 million years ago as I understand it. There is some uncertainty in the figures I have quoted (I am reciting from memory) and it not easy to know our position and motion with absolute precision. (well...if you're a mayan priest, perhaps you can figure it out.)

-V