You can hire seasonal workers by offering them a fixed-term contract of employment stating their dates of work. Should they sign the contract immediately, you will effectively be giving them unpaid leave for the first two months. However, you can’t remove their legal right to leave the job within those two months (having first given you the appropriate notice).
Alternatively, you could offer them a zero hours contract. This is more flexible, as it means that you can just give them work as and when it’s available, and only pay them for the work they do. However, they will have no obligation to accept the work.