Agnes, I found an old post which covers it.
With understanding the concept of time, I have found a simple calendar invaluable. I like this one as the spaces for the days are nice and big
organised Mum Home Planner
Here is what to do.
Start at the begining of a month. For about 3 weeks, simply get the child to cross off the relevant day of the month with a big cross from each corner of the box. Do it before bed and tell (sign and speech) them the day is finished.
They should start getting the idea of the page filling up (ie time passing).
You can then start putting in pictures depicting important future events (g parties, swimming, cinema trip,Christmas) What is nifty about this calendar is that it comes with lots of little activity stickers. I also make my own from Widgit Writing with Symbols package or Google Images or digital photos or drawings. Keep it simple to begin with.
Gradually (it may take months, that's ok, important thing is to do it every day)the child will realise that the event is coming closer. Also good for left to right skills and counting (e.g. birthdays, special holidays, family and friends visiting. Our big one at present is a skiing holiday).
You can then start to ask the child what happened that day when they are crossing off the day at night. Whatever they come up with is fine (eg even just something like 'sandpit') Honour their contribution and draw a little picture (or find a suitable one) to put in the box for that day and then cross it out.Even if the child is not able to communicate something, you can pick out something important from their day and put in a picture (e.g. for my kids today, it would be playing with the child minder's baby).
You don't have to have a picture for every day . Sometimes a cross may be enough. Equally though, put in the box anything of importance (eg for us it may be a tag from a special treat box of cereal or cinema tickets or a photo of something we made.)
You can then add a simple code to denote school/non school days. I use a red sticky dot (buy sheets of them in WHSmith)and put it in top right hand corner of every box which denotes a non school day.
If they start writing, you can transition to that (although even if they do, everyone loves pictures.) Ihave done this every night for 4 years and my children love it. Dd often goes to bed with a stack of calendars and flicks through them, enjoying reflecting on past events (eg holidays, the time she was sick on her sohes, the night a fox ripped open the rubbish bags, the days we went to the cinema.)