How colloquial or formal is "every other day/week/month"?

I was recently involved in a friendly yet heated debate over "every other week" and "every other month" being or not formal enough for contract terms and conditions, or being colloquial terms.

• "The parties agree to sync on the state every other week."
• "The parties agree on a grammar bull fight every other month."

As non-native speakers we both were not sure if this is sufficiently formal language.

Bonus question: If "every other month" and "every other week" are colloquial, what would be formal terms for a contract?

Solution 1:

"On alternate weeks/months" is formal enough (ngram).

However, the overwhelming superiority of the frequency of use of "every other week" makes me doubt that it should be really colloquial (ngram). As a matter of fact here are several instances of serious publications in which is found the term "every other week".

• Biotechnology and Biopharmaceuticals: Transforming Proteins … 2013 - Recommended dosage and monitoring requirements For RA, PA, and AS, the recommended dose is 40mg every other week. Some patients with RA not receiving methotrexate may benefit from increasing the frequency to 40mg every week.

• Handbook of Neuro-Oncology Neuroimaging Herbert B. Newton - 2016 - ... Maintenance Regimen Drugs Bolus Regimen CxT Regimen Bolus Regimen CxT Regimen Bolus Regimen CxT Regimen Methotrexate 10–15mg twice weekly (total 4 weeks) 2 mg/day for 5 days every other week (total 8 weeks) 10–15mg …

• Today’s Priorities in Mental Health: Children and Families S.H. Fine, ‎R. Knell, ‎T.Y. Lin - 2012 - We started seeing parents for an hour every other week; and when the children were 10 years of age, we were still seeing the parents for an hour every other week. There was no dearth of things to talk about. I don't mean that in most cases ...