Friday, March 20th marks the vernal equinox--the day on which Earth experiences a perfect balance. We'll have twelve hours of darkness and twelve hours of light.
In the northern hemisphere, we'll be moving into spring and summer, and so the light will grow each day (as it has been since the winter solstice, last December 21st). In the southern hemisphere, we'll be moving into autumn and winter, and so the dark will grow each day (as it has been since last December 21st).
Windows to the Universe offers great information, ideas, and lessons plans for introducing your students to the very global subject of astronomy. Lesson plans and resources are available in both Spanish and English. If you'll be teaching the vernal equinox, you can find information and diagrams suitable for beginning, intermediate, and advanced students.
National Geographic offers an activity with a fun scientific/cultural spin at A Reason for the Season on its Xpeditions site. Related activities for all grade levels are included.
Here's wishing you perfect balance! Happy Spring!