Lasagna is always better the day after it was made, i.e. as leftovers. It will be good for at least a week.
On a side note I just lost some milk. Poured some into my water bottle this morning. Noted the funny smell just before taking a sip. Comptemplated what I previously had in my water bottle that would leave such an odor while taking a sip. Realized that it was the milk with the funny smell as it was going down, since that particular water bottle was reserved for just water. Experation date: four days ago.