First Crossover is the final step before our Senate bills move (or "crossover") to the House. This is the last day for a final vote to be taken in the originating body before passing over. Also note that bills can crossover anytime before this deadline.
This is often-times a long day for Senators and staff (and likewise for the House members) as each bill is open for comment and debate. It is also an exciting time for those of us keeping track of our bills moving through the Legislative Process. If a bill passes through this stage, it is a quarter of the way to becoming law!