Why it works
Late timesheets hold up payroll and client invoices, and chasing them is nobody's favourite job. The reminder is identical every week, which makes it a perfect frozen recurring email — set it once and the nudge lands every Friday without you lifting a finger. Anyone who's stuck replies; everyone else just gets their hours in.
Make it yours
Send it a few hours before the actual cut-off so people have time to act. If your cycle is monthly, switch the cadence and the wording to match — the mechanism is the same.