The scenario is the whole office (30 people) is having an off-site day at park, like a (catered) picnic. It’s not a teamwork type of retreat, though I’m sure a side effect is better teamwork in the office. It’s a bring your kids, SO, etc, type of thing.
For freelancer who have only been in the office for 4 weeks, would not going be so bad? This is for someone having no kids or partner/plus-one.
Not going would just be a day off, as the office would be closed. To clarify, this is unpaid and cannot be billed.
Update: The question that has been suggested as already asked focuses mostly on how to make up the pay. The answers focus mostly on how to make up hours/pay. This question is focused on teamwork and socializing. The comment about whether or not it can or cannot be billed was added in response to questions, and it's a clarification not a focus.