The first day after the seeds sprouted. The seedlings are kept all the time in homemade mini-greenhouses made from plastic bottles.
On the third day, I noticed that the left sprout wasn't doing well and had slowed down in growth. I assume this happened because I forgot to make air holes in the bottle, and it was tightly pressed against the soil. I fixed it, and the next day, the sprout was doing well again.
I still haven't watered the soil; the sprouts have enough moisture from the greenhouse effect inside the bottles for growth.