I assume both guilds attack each other, and the guild that racks up the most points in their attacks win. (Worth noting that each day has a colour, which can affect the amount points you get). So on Day 1 Sparkle Ponies managed 140,881 points compared to Sith Lords’ 101,527, meaning Sparkle Ponies wins.
All the guild match-ups are decided on the first day (the day of rest that Sirrian mentioned)