Saying "till" doesn't make it clear if you're returning the morning of the 5th, or if the 5th is included in your leave. To be absolutely clear, you should state when you leave and when you return. I will be on leave October 4th and 5th, and I will return October 6th. This makes it clear which days you will not be in the office.
Possibly worth mentioning the commonly accepted - but incorrect - belief that the insertion of Julius and Augustus Caesar's months bumped the 7th-10th months up by two. Especially as the months that were replaced by July and August were 'Quintillis' and 'Sextillis' with obvious links to their 5th and 6th positions in the previous calendar.
Here is something I was able to discover on the internet the prime time I confronted the same predicament as you. 1st = primary 2nd = secondary 3rd = tertiary 4th = quaternary 5th = quinary 6th = senary 7th = septenary 8th = octonary 9th = nonary 10th = denary 12th = duodenary 20th = vigenary. These come from the Latin roots. The -n- ones come as well from Latin but this time are distributive ...
"on the 5th of November" is practically just removing the word day from the reference. As in "on the 5th (day) of November." It is used everywhere and even though it could be understood a few different ways it is the most correct. "on the 5th November" seems to me to more be dependent on the month and if not year. As in "it's my baby's 5th November" as in, the child is experiencing November ...