There is a poster at local Southern Stations colour-coded to indicate what service is running on each day over the festive period but for Monday & Tuesday this week, it says it might be affected by engineering works, not that it is a Saturday service.
Mind you, this morning, a ready reckoner poster has appeared detailing what the last trains are from Redhill and surrounding stations, which has clearly been produced by the local station team, but does the job. The website tells us it's a Saturday service with some peak hour extras, which aren't summarised anywhere.
Anyhow, service provision is more than adequate. With Thameslink as the mainstay of our services, there is very little difference between Saturday and Monday to Friday, except that in the evening rush hour, there are additional calls at Coulsdon South and Merstham on the 9Rxx's. Our driver usefully warned people they were being omitted on my train earlier.