Your baby pees once a day is he dehydrated?

There are some other issues to weigh into first:

What color is the urine? If it is a dark yellow color, probably dehydration. If it is a light yellow color, maybe. If it is more clear, probably not.

How much does he or she go when he or she does finally go?

Do you allow plenty of opportunity for your child to have a drink whenever he or she wants it, without waiting on your schedule? Leave a bottle or sippy cup of water around the house for whenever your baby just wants or needs a drink. He or she will crawl to it or reach for it when they are ready for a drink. Don't just rely on routine feedings of formula or milk with a bit of juice in between.

Does your child seem to struggle when he or she does try to urinate once a day?

If you are concerned your baby is dehydrated or is experiencing other symptoms along with urinating issues (or lack thereof), you should at least call the doctor's office, or better yet, visit the doctor to be sure.