You drink champagne at 6 to 10 degrees Celsius (depending on the Champagne). You drink some ‘oldies’ at 12 degrees.
It is best to put the bottles in the fridge a day in advance. Before serving, place the bottle in an ice bucket for 10-15 minutes, half filled with ice cubes and topped off 3/4 with cold water. Do not put too little water in the cooler, otherwise the bottle will be too cold. Also, not too little ice in the cooler, otherwise the bottle will not get cold enough. If you are too late to cool with the Champagne, I still have a tip: Sprinkle a good amount of salt in the ice bucket. Then you have a cold bottle within 5 minutes.
If you are going to serve the bottle without an ice bucket, keep in mind that a bottle will get warmer about 1 degree every 10-15 minutes. After an hour at room temperature, the bottle is therefore 10-14 degrees. At 14 degrees, the Champagne is really not good anymore. If you know in advance that you will be without a cooler (eg if you are going to have a picnic with a bottle of Champagne) put the fridge a little colder and take out the bottles at 3 to 4 degrees. You then have 2-3 hours to open the bottle at the correct temperature after removing it from the refrigerator.
Ice jackets that you put in the freezer beforehand work great for cooling a bottle. You have to keep an eye on whether the bottle does not get too cold (the same effect as with an ice bucket without water).
What you should never do is put a champagne bottle in the freezer. The bottle quickly becomes ice cold, but the Champagne is often still too warm. If you forget the bottle, the stuff also freezes and that is of course not good. Should this happen once: just put the bottle in the fridge and let it warm up slowly.