Never play without one
Remarkably, hockey was played for years before the mask was invented. The earliest versions were primitive in design, made of leather and fit flush on the goalie's face. It gave minimum protection and maximum pain when nailed by a 100 mph slapshot or backhander. Masks have evolved over the years from just a simple helmet and cage to many colorful painted designs and logos.
A goalie mask is a piece of equipment which no goalie would go on the ice without.
Goalies must get a mask that fits well as it's imperative they see the hockey puck. It's a lot harder trying to save a shot you can't see. The mask and straps should fit snugly so it doesn't wiggle and your chin should fit properly in the chin rest. You want a mask that gives you full protection, but isn't too heavy.
Goalie masks are basically helmets with shells made of durable hard plastic or composite materials, such as fiberglass or Kevlar, to add stiffness and strength. They have a replaceable stainless steel, chrome or composite wire mesh cage screwed to it for better peripheral vision and better facial protection. Many models have adjustable padding inside the helmet, to absorb any impact, and some come with goalie accessories like extended throat protectors.
Helmets and masks come in numerous sizes and colors and are usually measured in inches. If you are serious about your mask you can have them custom made and painted instead of just buying them off the rack.
Masks range in price from about $100.00 to $700.00. You can also purchase helmet/mask bags for about $15.00 to protect your investment. Make sure your mask is HECC certified.