4. (10 points) In a certain month, Friday, Saturday, and Sunday each have 5 days, so the 1st day of the month is a

保留了源文本的换行和格式。
【Analysis】First, based on the fact that 1 month can have at most 31 days, we can conclude: 1 month can have at most 4 weeks and 3 extra days; then, based on the fact that the month has 5 Fridays, Saturdays, and Sundays, we can conclude: the 1st of the month is a Friday, and solve accordingly.

【Solution】Solution: Since $31 \div 7=4$ (weeks) $\cdots 3$ (days),
so 1 month can have at most 4 weeks and 3 extra days,
since the month has 5 Fridays, Saturdays, and Sundays,
so the 1st of the month is a Friday.
Answer: The 1st of the month is a Friday.
Therefore, the answer is: Friday.