Newly hired and communication issues

I was hired at a local branch for a pretty big company. Before the holidays I was doing a preemployment physical and drug screen and was told I’ll probably start on 4 January, but this wasn’t definitive because they were waiting on the preemployment stuff to clear. But all their HR people have been on vacation for the holidays so I haven’t been able to communicate about when I’ll actually be starting. Do I just show up Monday at the start of business or should I call them Monday morning, because their HR people should be in? The holiday vacation and lack of communication due to it is causing me a lot of anxiety about my new job.

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