What is this "1 Month Subscription Sale" subscription, from XBOX? No information, no...

Could you explain further about what's bugged about it? Are you experiencing issues completing the purchase, is the correct price not being displayed, or is it all of the above? Xbox Sales and Specials should detail all promotions going on, if you're concerned
whether or not you're eligible for the $1 or 1 Euro promotion. https://www.xbox.com/en-US/promotions/sales/sales-and-specials

If that's not exactly what you're getting at, you could try to purchase the subscription online instead of on the console here on the Billing and Subscriptions page. https://support.xbox.com/en-US/browse/billing/xbox-subscriptions

You can also view and manage your subscriptions and billing on your console. For more information and clear details please follow this link.

https://support.xbox.com/en-US/my-account/manage-gamertag-and-profile/viewing-and-managing-your-memberships

The remaining balance will be deducted from what you pay for and give you whatever remains. So if you buy 1 month subscription but owe 1 month subscription, you will bave no subscription but will not owe anything. If you owe a week, you will have 3+ weeks
left (depending on how many says are in that month)

Yes. I have seen 1 month and 3 month subscriptions for sale at smyths toys and curry pc world see links. Link
1 ,Link 2

Hope it helps

A Gold subscription is the only way to play online. The 12 month subscription was just removed from Xbox, but you can still purchased 1 month or 3 month subscriptions or you can buy prepaid codes from other retailers (which is usually cheaper anyway).

If you redeem the prepaid code subscription and activate a gold family pack subscription, it will be added on top of the family pack subscription.. For every 2 months of regular subscription = 1 month for a family pack subscription.

a 1 month subscription will still be converted into a 1 month family subscription