In Episodes 7, 8 and 9 of Maria-sama ga Miteru (Maria Watches Over Us), they are set around Valentines Day. it's stated that at around this time Rosa Chinensis (Yōko Mizuno), Rosa Gigantea (Sei Satō) and Rosa Foetida (Eriko Torii) took their final exams and were waiting on their university selections which Sei got hers first and got her first choice and their graduations approching.
In Episodes 10 and 11 the events are set during Christmas where Sei's and Shiori Kubo relationship is revealed. in one scene Sei's and her petite sœur Shimako Tōdō are talking about Christmas and Sei makes reference to the upcoming exams.
When i saw the preview for Episode 12 it seems to carry on from the last part of Episode 9 with Yumi Fukuzawa and Sachiko Ogasawara are going on their date they had planned after Valentines Day.
I am wondering, what exactly is the timeline?