Translation:I will take a break today.
は doesn't specifically mark the subject of a sentence. は is known as the "topic particle" rather than a subject particle. The only reason は sometimes appears directly after a subject is when the topic of the sentence happens to be its subject, which often does happen to be the case. So when you see the は particle, you shouldn't automatically assume the word it's marking is also the subject, because it might well only be the topic.
- topic: 今日
- comment: 休みます
The verb of the sentence in the comment is 休みます (休む).
The subject of the sentence in the comment (the agent of its verb) is left unstated, since it is expected to be easily understood from context (or lack thereof) to be the speaker talking about hisself: わたし.
As for your suggested translation ("today is a holiday"), it would be translated into Japanese the following way:
今日は休みです。 ("[As for] today, [it] is a holiday.")
- topic: 今日 ("today")
- comment: 休みです ("is a holiday")
In this case, 今日 is the topic but is also the subject as well. The verb part of the comment's sentence is the copula です ("is"), and the complement this copula is linking is the noun 休み ("a holiday").
(More or less like that anyway.) ^^
In English, "I have holiday today" sounds pretty strange to me.
I would interpret that to be saying you have accrued holiday allowance at the company you work for.
Maybe you had used up all your annual leave the previous year, but today the next year has started and you now once again "have holiday".
I don't think you meant this though. What did you mean by it?
I think in some contexts your answer could work, but if you think of "yasumimasu" as "take a day off" it might be a little clearer why the future tense is better. You would use this Japanese to call in sick to work or some similar situation where you're not able to attend something that you were supposed to.
"I'm sick so I'm going to take the day off" rather than saying "I take the day off", "I'm going to rest today" rather than "I rest today".
Unfortunately, Japanese is slightly different from English from this sense. "今日は休みます。" contains the speaker's will, I 'am going to' or 'will' take a break. If it is present tense which means 'on-going' situation, then it is 今日は休んでいます。I am resting today. It is the different expression between when you will do something and doing something. i.e. 勉強します。means I will/am going to study. 勉強しています。means I am studying.