From Nencini:
"...the witness testimonies heard in the first trial that in the days preceding november 1st, 2007, nearing the due date of the monthly rent payment, filomena romanelli invited all of the girls to procure the sum of 300 euro that constituted the amount each of them had to pay for the payment of the rent of the apartment they occupied. it also results that the victim had the money at her disposal, and evidently kept it in her bedroom."
The rent hadn't been paid, which presumably was because of the extra long holiday weekend and nobody being available to accept it. But we know Guede was under a great deal of pressure over his own unpaid rent and urgently needed money.
Ron Hendry speculates:
"Rudy Guede had no visible means of financial support and had been active of late as a burglar – Being the first day of the month, Rudy’s rent was due as was rent for others in Perugia. Being a long holiday weekend, many would be gone and some would be expected to have rent money stashed in their apartments. Thus, Rudy had strong motivation to locate an empty apartment for burglarizing."
If the money was going to be there on the 31st, it would likely be there on the 1st, 2nd, 3rd and 4th for payment on the 5th, as a result of the peculiar circumstances of November 2007. And 1st November was a quiet night in Perugia. The 31st was not.
But whether Guede chose the right day or not in any case does not detract from his very clearly defined motivation.