" I can't use paragraph style with Next Style"
Having seven paragraph styles, one for every weekday, that have the "next style" set accordingly, seems to be the best solution.
I assume that you confuse next style with GREP style. GREP styles are inside the same paragraph. With next style you need several styles in succession in the same story.
As Gerald wrote, you have to create 7 Paragraph Styles, each with a Next Style defined. Select all Paragraphs and choose the first paragraph's style with the right mouse click and apply this style and next style.
Perhaps this is a table, in which case next style won't work, but Table and Cell Styles would...
I am using grep instead of next style because I am linking InCopy file to different indesign files. I this case the indesign pulls styles set in Incopy soo i have to change avery paragraph style (7 different paragraf with too many details). In InDesign it does not override the "based on" and "next style" if it was sat in InCopy.