You answered your own question. Your first day is on a Monday and so are many, many other visitors to WDW and most head for....you guessed it...MK on Mondays.
We usually avoid MK on Mon and Sat, but if you have to go on these days I suggest going as early as possible and just utilize FP. Quite honestly MK is always the busiest park and it's still busy other days but not as bad as Sat or Mon.
October can actually be a slower month. The resorts are very busy because of conventions.