Did you know that people generally tend to become more agreeable as the work week goes on? According to Psychology Today, it's true. While we may feel as though we're getting increasingly irritable and decreasingly cooperative, oftentimes we're actually becoming more agreeable and accommodating (perhaps to just get our boss off our back...).
Bottom line, what this means is that if you have any control over when a negotiation is taking place, try for a Thursday. It won't guarantee you a raise; but, it may present conditions that are most likely to work in your favor.
To build this out even further, pay special attention to the rhythm of the day and week for your boss and anyone else who will be included in the conversation. Prioritize your salary conversation for when they are unlikely to be distracted, will not be running to or from meetings, and will have the best chance of being able to focus on you.