I think it all started with "partiu" and only afterwards the trend of using weekday names in the past/tense came to be.

There are two things to understand here:

1 - the past tense is used here to imply that it already happened and only you didn't realize, it's creating a sense of urgency and given the person who hears/reads it no time to think if he/she really wants to do said thing (no one wants to be left out or be seen as the one person who didn't take part). It's a persuasion technique and, while everyday people use such "words"/hashtags, truth be told they are first and foremost marketing tactics.

2 - The act of using nouns as verbs is something we are copying from English speakers.

That being said I find it quite distasteful and bad use of language. it's a shame people don't realize the persuasive nature of such trend.