Using relevant position in a vector to find index

I have a vector for each participant in a study that describes their eating periods. For example Participant 1 and 2 respectively have

> eating.period.1
 [1] "Day 1 (1/18/15)""    Breakfast""    Morning Snack""    Lunch""    Afternoon Snack"
 [6] "    Dinner""    Evening Snack""Day 2 (1/19/15)""    Breakfast""    Morning Snack"  
[11] "    Lunch""    Afternoon Snack""    Dinner""    Evening Snack""Day 3 (1/20/15)"    
[16] "    Breakfast""    Morning Snack""    Lunch""    Afternoon Snack""    Dinner"         
[21] "    Evening Snack""Average"> eating.period.2
 [1] "Day 1 (9/16/15)""    Breakfast""    Morning Snack""    Lunch""    Afternoon Snack"
 [6] "    Dinner""    Evening Snack""Day 2 (9/17/15)""    Breakfast""    Lunch"          
[11] "    Afternoon Snack""    Dinner""    Evening Snack""Day 3 (9/21/15)""    Breakfast"      
[16] "    Lunch""    Dinner""    Evening Snack""Average"

How do I get the index for an eating period between Days X and X+1?