An alien from the planet Math came to Earth on monday and said: A. On Tuesday he said AY, on Wednesday AYYA, on Thursday AYYAYAAY. What will he say on saturday?
1. Let's denote the sequence on the \( n \)-th day as \( a_n \). We are given the sequences for the first four days:
   - \( a_1 = A \)
   - \( a_2 = AY \)
   - \( a_3 = AYYA \)
   - \( a_4 = AYYAYAAY \)

2. We observe that the length of each sequence doubles each day. Specifically, if \( S(a_n) \) denotes the length of \( a_n \), then:
   \[
   S(a_{n+1}) = 2S(a_n)
   \]
   For example:
   - \( S(a_1) = 1 \)
   - \( S(a_2) = 2 \)
   - \( S(a_3) = 4 \)
   - \( S(a_4) = 8 \)

3. The pattern in the sequences suggests that each new sequence is formed by taking the previous sequence, appending a space of the same length, and then filling these spaces with the previous sequence in an alternating order. Let's verify this with the given sequences:
   - \( a_2 = AY \) is formed by taking \( a_1 = A \) and appending \( Y \).
   - \( a_3 = AYYA \) is formed by taking \( a_2 = AY \) and appending \( YA \).
   - \( a_4 = AYYAYAAY \) is formed by taking \( a_3 = AYYA \) and appending \( YAAY \).

4. To find the sequence for Saturday, which is the 6th day, we need to continue this pattern:
   - \( a_5 \) will be formed by taking \( a_4 = AYYAYAAY \) and appending its alternating sequence:
     \[
     a_5 = AYYAYAAY \text{ followed by } YAAYAYYA
     \]
     Therefore:
     \[
     a_5 = AYYAYAAYYAAYAYYA
     \]

   - \( a_6 \) will be formed by taking \( a_5 = AYYAYAAYYAAYAYYA \) and appending its alternating sequence:
     \[
     a_6 = AYYAYAAYYAAYAYYA \text{ followed by } YAAYAYYAAAYAYAAY
     \]
     Therefore:
     \[
     a_6 = AYYAYAAYYAAYAYYAYAAYAYYAAAYAYAAY
     \]

5. Thus, the sequence the alien will say on Saturday is:
   \[
   a_6 = AYYAYAAYYAAYAYYAYAAYAYYAAAYAYAAY
   \]

The final answer is \(\boxed{AYYAYAAYYAAYAYYAYAAYAYYAAAYAYAAY}\)