If you came here for the restaurant that is known for it's abuse of the apostrophe, T.G.I. Friday's, then you've come to the wrong place. We don't abuse innocent apostrophe's here.
edit Why Friday?
The first thing that springs to mind is why we are looking to thank any deity at all. Assuming for a minute, beyond any evidence to the contrary, that there is an interventionist God, why would anyone thank them for Friday?
Doesn't time continue moving constantly? Isn't one day followed by another? If it is, then no intervention is needed to get to Friday. One can spend the other six days of the week praying that Friday will appear, and there's an extremely high likelihood that it will - much the same as if one spends the previous six days not praying.
And why Friday especially? Of course, it's end of the work week, yadda yadda. But Friday is still a work day. Why not Saturday? After all, if Friday is exciting because it means the last day of work for the calendar week, wouldn't Saturday being a day of rest be more exciting?
Of course, I wonder if we don't get excited about Saturday because once it arrives we're disappointed. "Hooray, It's Saturday. Now I have a day when I have to do all the cleaning that I didn't get a chance to earlier in the week. Plus I have to mow the lawns, fix the car. Oh, and the in-laws are coming to visit... How long until Monday now?"
edit Why God?
But, assuming that Friday is the most exciting day of the week, and assuming that we have only gotten there due to the intervention of a deity - ignoring the suspension of disbelief - then which God are we thanking?
Now many in the western world will automatically assume a Christian God. This is a false assumption. Basing it on the whole of the phrase, the Christian God is only assumed due to the lack of name. (After all, we don't say "Thank Allah...", "Thank Jehovah..." or "Thank Buddha...".)
Now it is suggested that we are looking at a single deity, by the use of the singular. But that doesn't automatically assume that we are looking at a mono-theistic concept.
What we can assume is that it is a deity that intervenes in a direct way with mankind - otherwise why would we need to give thanks?
So the only identifier we can go by to really narrow this down is the specific in the statement - that it is Friday.
edit Who again?
So, of all the Gods in belief systems universal, which God has the most to do with it being a Friday?
Well, in English we call it Friday from the old English frīgedæg, after the Goddess of Love - Frigg (AKA Frige, Freyja, Freya, Freja, Freyia, Frøya, and Freia).
In Latin, Friday is known as dies veneris. This literally translates as venereal day.
And thinking further on this, Friday is often considered "date night". We're often encouraged to wear casual dress in the corporate world, which allows us to show ourselves in the most flattering way to our work colleagues, in the hope we might be able to get a quick fondle in the supply cupboard.
This means that Friday is more related to Goddess of Love, Frigg, than it is to any other ethereal figure.
Many religious creeds suggest we shouldn't be using the name of God in vain. Secularism and political correctness suggests that we should avoid using God in places where it's not appropriate. So for the point of accuracy and causing the least offence, the saying should be changed.
So, from here on, it should read as saying "Thank Frigg it's Friday" as the more polite version of the phrase. Ensure when you are next at work, school, or any other social engagement on a Friday, that you greet people with such.
They will be amazed at your politeness.