Can onsite Contractors work at a government agency/facility with no onsite government personnel present?
The short answer is there is no written regulation that requires a government employee to monitor contracators working on site. The contractual relationship is no different from a grounds maintenance contract where the contractor performs generally with minimal Government oversight, so Government presence is not a requirement.
Lets make some assumptions to narrow the scope of the question:
1) the contractor employees are performing on a non-personal, performance based commercial service contract;
2) there is no language in the PWS addressing the Government will furnish a work site or that the contractor will perform at an alternate work site if the Government site becomes unavailable;
3) this is not for performance of classified or similarly sensitive work-product where Agency and/or local security policy direct Government employee presence;
4) the CLIN description or place of performance (section F of UCF) stated in the contract is the Government facility that will be closed for the holidays.
A review of FAR Part 37, DFARS Part 237, AFFARS Part 5337, and AFI 63-138 does not reveal any relevant guidance. Office of Federal Procurement Policy (OFPP) letter 93-1 is similarly silent, only establishing policy to ensure "Sufficient(ly) trained and experienced officials are available within the agency to manage and oversee the contract administration function". I could find no directive for government employees to be in a government work-place during performance.
Now if the government was required to be present in order for the contractor to perform, the absence of federal personnel would likely be considered a Government caused delay pursuant to FAR 52.212-4(f) (or FAR 52.249-14 if non-commercial). The place of performance is the Government location, and the Government is not making the location available. This could reasonably be considered a delay beyond the control of the contractor as an act of the Government in its contractual capacity. The most common solution is for the contractor to have one person able to open the building or work space and they perform to their PWS (accomplishing PWS mandated training such as cyber-awareness, and other organizing/administrative tasks was a frequent attention of the contractor). The COR would then review the work product upon their return.
In most cases I've observed, the contractors opened the space, completed theirr work, and closed the space.