For any ESL folks here -

"Friendly reminder" is typically used for reminding people of common knowledge. Especially for beneficial but inconvenient things that some or most people neglect to do, either because they're annoying, inconvenient, or time consuming. Things for which busy people might need a "wink wink, nudge nudge".

Friendly reminder to floss. Friendly reminder to have your cancer screening. Friendly reminder to check your tires. Friendly reminder to file your taxes early. Friendly reminder to drink more water, eat fiber, etc.