"because it's" and "as\since its"

0Hi,02br
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00I am new here, and apologise if this is an easy question, or it has been answered before. I did a quick search and found some interesting ideas on the differences between "since" and "because", but did not find anything that helped resolve my uncertainty. Too, I am well aware of using the conjunction "it's" and the possessive "its".02br
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00Here is my problem. I was writing a note via email to my wife as follows:02br
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00"Kim, tonight we are celebrating the new deal over an unofficial drink as it's Friday."02br
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00I was surprised to find that my grammar-checker flagged "it's", suggesting it should be "its".02br
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00This caught me off guard because what I was trying to say was:02br
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00"Kim, tonight we are celebrating the new deal over an unofficial drink as it is Friday."02br
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00As it turns out, this last sentence does not cause grammar flags, and so I assume it's correct, if not elegantly formed.02br
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00So I started wondering why the first sentence was wrong.02br
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00Well, it occurred to me that using "as" (or since), the grammar is treating "Friday" as a property of the day referred to by "it". Fine, but that still does not explain why both "as it is" and "as its" work, but NOT "as it's"!!02br
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00I did some experimenting and found that if I wrote:02br
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00"Kim, tonight we are celebrating the new deal over an unofficial drink because it's Friday."02br
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00I also get no grammar mistakes.02br
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00So my question is simple (perhaps): Which way is right? And if both are, then why can't I use "as it's"??02br
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00I imagine this is all simply because my grammar-checker program is not perfect, but I would like to know if there are differences, and if a correct-usage exists for me to learn by this affair.02br
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00Will0-