Yes, it is correct to say 在美国 for 'In America'. You just need to add 日： 在美国, 明天(7月4号)是节日。 Or you can say 假日 or 休息日. Both words mean you don't have to work on that day. 节日 is more like a festival, a day to commemorate or celebrate. People might also say something like: 明天放假。 In Chinese, people don't use nouns as often as in English. For example, She is a good student. would be: 她学习很好。 He is a good cook. would be: 他做饭做得很好。

You should probably say "在美国, 明天是节日" If you said "在美国, 明天是节" to a Chinese, he would definitely be unable to understand it, because in Modern Chinese, the character "节" cannot be used as a WORD to mean "holiday" or "festival". It only exists in words consisting two or more characters, such as 春节 中秋节 过节 节日

Holiday means "节假日，假期". So 在美国，明天（7月4日）是节假日 is right. But we will often say, 在美国，明天（7月4日）放假. Or we can also say that 明天（7月4日）是美国的国庆节.

in China they say国庆节Guóqìng jié ; for July 4th I think you can say the same

在美国，7月4日是独立日。 耶！