Final answer:
Based on the text provided, the tickets to the Mariners game that McTeague purchased were for Monday night.
Step-by-step explanation:
The dialogue from the text suggests that the character named McTeague is interested in finding out what day the tickets he purchased are for. McTeague asks the seller if the blue slips of pasteboard, which are likely the Mariners game tickets, are for Monday night. This indicates that the day of the week the tickets are for is indeed Monday.