Not even the best negotiation tactics can overcome the folly of asking your boss for a raise at the wrong time. That's why doing a little recon work might keep you from fighting an uphill battle.
Depending on your schedule, your work week will have consistent peaks and valleys. There are probably times of the week when you're more stressed, like Monday mornings, or times when you're more relaxed, like Friday afternoons. Well, the same goes for your boss. If you want to time your raise request as best as possible, Ted Leonhardt at Fast Company suggests you find out when your boss is usually feeling good and plan to chat with them then:
...the simple lesson here is that your timing strategy needs to take your manager's emotions and personal quirks into account. When people are happy, they tend to be more generous. Look for a moment when your superior is feeling good in general and good about your work in particular.
If your boss is usually relaxed and in high spirits on Friday afternoons, aim for that time slot. There are plenty of other things to take into account regarding your timing, but your boss's mood is a big one, so don't brush it off. Asking for a raise is already hard enough.
How To Perfectly Time Your Request For A Raise [Fast Company]