2023 GCHQ Christmas Challenge

Explanation:

Clock 1 is 8:00pm, or 20:00. The 20th letter of the alphabet is "T".
Clock 2 is 9:00am, or 09:00. The 9th letter of the alphabet is "I".
Clock 3 is 1:00pm, or 13:00. The 13th letter of the alphabet is "M".
Clock 4 is 5:00am, or 5:00. The 5th letter of the alphabet is "E".

Explanation:

The day breaks every morning; every four years a leap day is added in February; the adage "carpe diem" or "seize the day"; we refer to the here and now as "present day".

Explanation:

I have no idea, these words strike me as especially British so I assume I'm missing some important context that would help solve it. I reverse-engineered the word from its use in the final challenge.

Explanation:

E = 0
M = 1
I = 2
T = 3
A = 4
S = 5
R = 6
O = 7
D = 9

Explanation:

Sequence 1 is ordinal suffixes, descending: (4)th, (3)rd, (2)nd... (1)st
Sequence 2 is the word "COMPLETE" spelled backwards
I am not sure what the meaning of "WU, SQ, OM" is but from the others I deduced that the next is "KI"
Sequence 4 is the word "WRAPPING" spelled forwards

Explanation:

I have no idea about this one. Reverse-engineered the word from its use in the final challenge.

Explanation:

It's a straightforward case-insensitive substitution cipher:
    A = F
    B = N
    C = A
    D = O
    E = D
    G = I
    H = L
    I = B
    K = R
    L = S
    M = E
    N = C
    O = H
    P = K
    Q = T
    S = U
    U = V
    V = W
    X = Y
    Y = M

Numbers and punctuation are not substituted. Letters F, J, R, T, W, and Z are unknown since they weren't present in the code. This decodes the following message:

    First solve the code.
    Identify a 4-letter word.
    Look everywhere.
    Maybe it's very obvious.

The first letters of the decoded lines spell out the answer.

Explanation:

In each box is a picture and a number. The picture represents a texture
that corresponds to the Christmas item shapes behind each challenge
number. The number represents the letter of that challenge's answer word
to write in the box. The image/shape matches weren't all obvious to me,
but I worked out that this is how they map:

    3 - Gingerbread (Challenge 2)
    2 - Present (Challenge 6)
    3 - Mitten (Challenge 7)
    4 - Ornament (Challenge 1)
    1 - Candy Cane (Challenge 4)
    6 - Cookie (Challenge 5)
    1 - Gingerbread (Challenge 2)
    3 - Candy Cane (Challenge 4)

    1 - Mitten (Challenge 7)
    4 - Candy Cane (Challenge 4)
    5 - Bells (Challenge 3)
    2 - Mitten (Challenge 7)
    1 - Bells (Challenge 3)
    2 - Ornament (Challenge 1)
    2 - Cookie (Challenge 5)
    2 - Gingerbread (Challenge 2)
    1 - Ornament (Challenge 1)
    5 - Present (Challenge 6)
    4 - Bells (Challenge 3)
    6 - Present (Challenge 6)
    1 - Cookie (Challenge 5)