We go for the one with vertical columns as moondog suggested so that DS can easily do a count down to events like holidays, end of term, school trips etc. It's the vertical one that's running low on stock.
I know there are similar products (cheaper too!) out there but if it ain't broke I ain't gonna fix it.
The stickers give a nice visual cue for non-readers & readers alike (there have been times when I've got sick of making my own aids/writing social stories etc, etc so for me ths product is a lot about convenience as much as anything else lol!)
The more mentally prepped he is for thing like this, the lower the incidence of meltdowns due to unexpected routine changes iyswim.
If you do a search on these calenders and "moondogs clock" idea you'll see just how helpful the main family calender can be for HFA/Aspie kids.
Over the course of the year DS has finally gained a marginal sense of time elapsed using this calendar & a clic loc watch (brilliant for dyspraxics, arthritics etc!). I'm coming year will keep developing this nascent skill. DS just marks off each day before bed every night. His AS 40+ Dad has NO sense of time elapsed and I cannot describe how irritating it is.
For me, I would have lost track of all DS's & my siblings appointments without the darn thing this summer - being dyslexic I NEED help remembering all these appts! Ours hangs in the hall by the front door so I can easily see any important appts every morning. A scatty Mum and an ASD son can be a VERY bad morning combo without a little help 
This year I've ordered a children's calender too, to try and get him to start organising his own life a little bit. It's an experiment, & I suspect may be too ambitious, but you gotta try right? I can see this or one of the diaries perhaps being a godsend at for teens/adults though?