Unlike Job Seekers Allowance where you have to wait until Monday to apply because Job Centres and DWP offices are closed over the weekend, Universal Credit is claimed online. The World Wide Web is never closed and Universal Credit is calculated over a calendar month. The day your application is submitted is the 1st day of your claim. If you delay making your application until Monday you will lose out on those 2 days payment in addition to the first 7 non entitlement days (waiting days). Universal Credit cannot be backdated. If you rent your home you will also not receive any help with your rent for this period.
In most areas in Wales this affects single people newly unemployed. If you live in a Job Centre area where Universal Credit is live and you do not have sufficient National Insurance contributions to claim Contribution based Job Seekers Allowance, you will have to apply for Universal Credit instead of Job Seekers Allowance.