A hollandaise sauce is only good for 4 hours. You can keep it warm and use it that way. When you put it in the fridge, it will turn lumpy. This sauce is made from eggs and you shouldn't try to keep it, for any amount of time.
I keep leftovers of it overnight in the fridge for about 24 hours. Just add a little hot water to it and stir when your ready to eat. It's safe if you follow basic cross contamination rules. If you have made it yourself with butter and eggs it shouldn't be lumpy after stirring in hot water.