You have already broken the problem down into pieces. Tackle the pieces one at a time. Each piece is not so difficult once you decide on hardware.
Ability to download a 12 month temperature variation + day/ night settings to a controller
How large is the data file? Perhaps store on a SD card? You will need to learn to interface the card hardware and understand the software to access the card.
Control a heater up to a maximum load of 2 kW
Learn to control mains powered device. Maybe solid state relay?
Display to show the variables
Decide on a display. 1, 2, 4 line LCD, graphic LCD? Again learn interfacing and software.
Sensor for both inside and outside T°C
Decide on temp sensors and figure out how to read and convert the output to degrees C. DS18B20 is very popular and you can hang a bunch on a one wire bus (something else to learn).
And so on. Once you have the pieces study Finite State Machine. Use a FSM to build your program from the pieces.