Since when people began to celebrate Christmas as a holiday?

What's the main difference between saying "greeting" and "greetings?"

Good question! The plural form "greetings" is used as a word of welcome or hello. You wouldn't use the singular "greeting" to greet someone. "Greeting" in the singular form can refer to a hello, or it could be the present participle or gerund form of the verb "greet." Ex: Greetings, friends! How are you all? Ex: I am going to be greeting so many people today at the event. Ex: What a lovely greeting she gave us.

Why does he say “redheads”? What does it mean?

"Readhead" refers to someone who has red hair. Some people with red hair are fine with this term being used, and some do not like it. To make sure you are being respectful, it is best to say "someone with red hair" if you need to identify someone by their red hair color.

When can we use infection in plural form? Or is this speaker’s mistake?

In this video, it actually should be "the number of infected." The word "infection" is the singular and plural form of this word. You can use the word infections when you are discussing different types of infections, like a staph infection or a mrsa infection. You can use the word infected when discussing the past or a number of people who are infected, like the news anchor did in this video. Here are example sentences: Ex: He has a tooth infection. Ex: There are thousands of people infected with this new virus. Ex: She almost died due to multiple infections in her body.

What does "magic words" mean? I don't think it literally refer to a magical spell!

You're right! "Magic words" doesn't refer to a magical spell. "Magic words" refers to polite words. "Magic words" will encourage someone to do something as if they were magic. Growing up, children are taught to say the "magic word" "please" to be polite when asking for something, rather than just demanding something. Some other "magic words" would be "thank you," "you're welcome," and "I'm sorry." Ex: I won't help you unless you say the magic word. Ex: Lilly forgot to say the magic words "thank you" when she received the gift.

What's "day job" mean? Like opposed to "night job?" Is there a "night job?"

A "day job" is a person's regular job or main source of income. It has less to do with the time of day and more to do with the fact that it is done every day in order to earn money. When you are not doing your day job, you could be trying out a more interesting occupation (like making candy floss) or following a hobby (like painting). There is no such thing as a night job, but you can say "night work" or a "night shift" to say that you are working at night. Ex: Mick has traveled around the world for 30 years and has never had a day job. Ex: His day job was simply a way of paying the bills!