Does in review mean an external reviewer is assigned ?
'Under Review/In Review' can mean that your manuscript is currently assessed or 'under review' by the assigned editor. This could be very likely, since you mentioned the status changed straight from 'waiting for assignment' to 'in review'. During this time your manuscript could be checked for formatting guidelines, the editor might judge the article himself or he might be in the process of trying to find suitable reviewers. For details on the different status messages of journal's workflows, here's a great answer.

Especially around holidays it might additionally be hard to find reviewers, and get back their scores within a certain time period. Also, even if a journal claims to normally take one month until the first decision, I suspect these times can have a big variance.

I suggest to wait until the holiday season has passed, and then follow up if the status has not changed yet.