"Whoever wants to be my disciple must deny themselves and take up their cross and follow me" (Mark 8:34).
Denying ourselves is both an Old and New Covenant instruction. This verse in Numbers adds, "do no work". It's surprising how much we struggle with a God who tells us to stop work, even for a day. You would think that God telling his people to rest would be cause for delight, but we find the Israelites often disobeyed this commandment.
There are many reasons for this. Through work, we gain a sense of pleasure and feelings of success, and not working denies us this enjoyment. Often our self-esteem is connected with our achievements. We feel good about ourselves when we consider our accomplishments, but no matter how much we achieve it will never feel like it's enough. We have to keep achieving to maintain our self-esteem.
Stopping work is a way of denying ourselves. It reminds us that the world will continue quite happily without our contribution. It makes us realize we aren't self-sufficient as we'd like to think, and it causes us consider other worthwhile things we could do with our time.
Many world religions are wholly focussed on fasting, denying of pleasures, and the serious pursuit of an ascetic lifestyle. Whereas Christianity, although it includes denying yourself, also includes much feasting and celebration. Our celebrations around Christmas and Easter are often about eating special treats—turkey, pudding, chocolate, hot cross buns.
Denying ourselves is an attitude that we develop as we realize our need for self-esteem is met in God. Therefore we don't need to be focussed on ourselves, and we can take time to celebrate.