There are some tricks that help remember the "Number of Days" competition. Assuming that they entered the house on a Wednesday
Evictions and HOH competitions generally happen on Thursdays. (Days 9, 16, 23, 30 etc)
Nominations, Road Kills happened on Fridays. (Days 10, 17, 24, 31 etc
VETO Competition happened on Saturdays, (Days 11, 18, 25, 32 etc)
VETO meetings happen on Mondays (Days 13, 20, 27, 34)
Obviously would not help for special days like the day of the Carnival (Jesse) happened.
What I would like to know if there was a previously evicted houseguest who was out of the house for a considerable amount of time(like Glenn) would they tailor the challenge so it would not put him at a competitive disadvantage?