Why I can't I book a date for my UK Visa appointment?

I am planning to go to UK in the mid of December for a visit. Currently, I want to book for my visa appointment on early of November, however, I can't click any date in November. The only available ones are to book a date in October. I wonder that how it would be possible that there still are so many available options to book a date in October and no options at all in November. Can anyone enlighten me about this? Thank you in advance.
Best Answer
In my experience of making two applications, the furthest in advance you can book is around 21 days. Both times I’ve booked an initial date just to get my application into the system as early as possible, then I’ve changed the booked appointment date online as soon as further dates become available.
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Which dates are available for UK visa appointment?
Booking Appointments You can book an appointment up to 28 days in advance. UKVCAS offers a range of non-chargeable and chargeable appointments. Non chargeable appointments are mainly available between 09:00 and 14:00 Monday to Saturday.How can I book my visa appointment date?
Everyone else, follow the steps below!How do I book an appointment for UK visa?
Use this step-by-step guide to help you with your visa application.When can I reschedule my UK visa appointment?
Rescheduling an appointment: You are allowed to reschedule your appointment 1 working day prior to your scheduled date. Please note that you can only reschedule your appointment up to a maximum of 2 times. Further change in date will be considered as a fresh appointment and a new service charge will be applicable.Liz Phair - Why Can't I? (Official Video)
Sources: Stack Exchange - This article follows the attribution requirements of Stack Exchange and is licensed under CC BY-SA 3.0.
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