Depends on what time, for me 5PM is when all prizes for ELO are given out, so anything before that time is reset. But anytime after that the ELO value will remain the same until next Sunday, it just depends on your timezone really.
Here are the deductions from what I think are true... XD
Miss a day: -2 Elo
Miss 2 days, 3 days, etc. -4, -6
Miss a certain number of battles you were supposed to do- A big chunk XD
And I understand your question... If you're Elo value on Sunday is 1300, it won't drop since Sunday is considered the last day of Elo (I think XD) However, if you don't do the NECESSARY battles, it will drop sadly...