For the meaning of Eminent - expansion, harvest - determination click HERE
Si-day is the day of the snake in the Chinese calendar. If you want to change something, a day or year of the snake might be a good moment. If you want to know which days are days of the snake, then buy a Wan-Nian-Li in a Chinese bookstore. It is a calendar. Once you know the character Si (looks like an S) you can easily find all snake-days. A WanNianLi is cheap and it lasts about 100 years.
Line 3. 'Set things right' might also be: marching (too) correct, or: to go on a punishing expedition.
Line 6. 'Set things right' or go on a (punishing) expedition or to go on a trip.