Could be something draining the battery. Or, since the battery was new yesterday maybe it was never fully charged if you didn't run the tractor very long? A lead acid battery loses around 50% of it's power going from 72 degrees down to zero.
Take the battery inside and warm it up to room temp. If you get the tractor started later, after shutting it off for the day, disconnect the positive cable until you want to start it again. If it's a battery draining thing, you will prevent that. If your charging system is working, you should see around 14.5 volts across the battery terminals with the engine running.