For fixed term contract, you will have to look into the contract about annual leave.
For those working only on weekends I don't think they are entitled to annual leave. Most probably such worker are paid by the day and as such calculation of ORP for items such overtime shall be similar to daily rated workers If they paid by month, then you may use 26 days like those monthly rated workers.
KL Siew has put up a sister site by the name of mylabourlaw.net, which is in effect a more user-friendly version of the present popular pesaraonline.net. The site is in the process of being fine-tuned but it is fully functional. You are invited to visit the website at http://www.mylabourlaw.net and give your views or suggestions. Queries can be posted and be responded from there as usual.