The first place to look is your tenancy agreement, as it is likely to contain restrictions on your ability to sublet your home.
There may be an outright prohibition on subletting, which means you cannot lawfully sublet. More likely though, a fixed-term tenancy agreement will allow you to sublet, subject to you obtaining the landlord's consent. The landlord will be unable to refuse consent if the request is reasonable.
If you unlawfully sublet your property without the landlord's consent, you risk being in breach of your tenancy agreement and the landlord could take action to evict you.
The position is more complicated if you are a landlord and you want to evict your tenants to let out your home for Christmas.
If the fixed term under the tenancy agreement has expired, you can generally serve what is known as a Section 21 notice to evict your tenants on a 'no fault' basis. However, your tenants must be given at least two months' notice so it will not be possible to let your property out for Christmas. Even then, if your tenants do not leave by the specified date, you may need to apply for a possession order.
Earlier this year, the government announced a plan to ban 'no fault' evictions which will make it even more difficult to evict tenants.