Before you get started, you need to know your hardiness zone as a starting point before growing. Here in the lower mainland, we hover around zone 8. While this is a useful number, it is not the only thing that dictates when or what to grow. Within an average suburban backyard, you are going to potentially have a number of different microclimates. You will need to take into account sun, rain/moisture, soil and wind exposure, as all of these elements are going to affect what you can grow in any given square meter. For example, when the snow falls and the ice melts in my yard, there are areas that stay frozen for much longer than others. Other areas might get little to no snow or ice at all.
The days to maturity indicated on a seed packet are based on optimal conditions. You can expect that your harvest will be delayed (or non-existent!) if your beds are located in a spot that is not ideal for that specific plant. I have fallen into the trap of counting down the days to maturity on a plant only to be disappointed. You can't cheat the system, the plants know if they're not getting what they need! Where I have situated my beds such that they get full sun exposure and they are protected from any biting winds. It also helps that they are raised bed so that they have good drainage and the soil structure and content is easily amendable.
If you are in Canada, I find the information provided by Vesey's Seeds helpful. It shows a hardiness zone map and lists popular AVERAGE (not definitive!) first/last frost dates by province. Again, you can use this information as a guide to determine which plants to include in your garden, when to sow your seeds, and how to protect plants which are marginal in your area.
If you are gardening during the traditional growing season in your area, the first frost date will determine when you should start sow seeds indoors and direct sow in your beds. I find it helpful to have a separate calendar for gardening each year. You can grab an inexpensive one at a dollar store or print your own from an online template. I have my first frost-free week boldly marked in my calendar as if I'm counting down to Christmas! Each week prior to the frost-free, I have it marked to indicate 'frost-free week -1,' 'frost-free week -2,' 'frost-free week -3,' and so on. I list each task I want to accomplish in these weeks leading up to my frost-free date, including the seeds I need to sow and whether they are indoor/outdoor sown. When using seed packets or planting charts, I can quickly and simply pencil in the type of seed on the calendar depending on when it needs to be sown relative to the frost-free date. It sounds more complicated than it is in practice and I find it saves a lot of time to have everything in one place for reference. You probably also find that you have a little extra time on your hands in the dead of winter as you stare longingly through your windows as you plan your garden. If you are really keen in your planning and love taking notes (ahem), you might consider making a small investment in a garden journal which helps you track everything including activities such as sowing, harvesting, and even budgeting.
Below is are two photos of my brand new polytunnels, taken less than 24 hours apart. Oh, what a difference a day makes.
Ok. Now that we've got the basics sorted out, the fun can begin!