At least one of arrays nodes must be filled If all arrays nodes are empty, abort validation.

const validator = Joi.array().items(Joi.object({
    day: Joi.string().required(),
    nodes: Joi.array().items(Joi.array().items(Joi.number()).length(2))
})).length(7)

const week = [
    {day: 'monday', nodes: [[5,12]]},
    {day: 'tuesday', nodes: []},
    {day: 'wednessday', nodes: []},
    {day: 'thrusday', nodes: []},
    {day: 'friday', nodes: []},
    {day: 'saturday', nodes: []},
    {day: 'sunday', nodes: []},
]

Thanks