Every planet in astrology has its own distinct energy, and impacts us in different ways. They have their place in your natal chart, and they have their impact via transit. Which planet does the initiating of them all? That would be Mars.
Mars in Astrology
In your natal chart, Mars shows what energizes you, drives you to take action, and how you act when you do. This is mostly seen through the sign and house location of your natal Mars.
Transit (moving) Mars impacts your natal chart by touring through your natal houses and aspecting your natal planets. This impacts where your energy and drive is focused currently.
Take Initiative With Mars
So, times that are good for taking initiative usually involve Mars. When you have the transit planets making a beneficial aspect (sextile or trine) to your natal Mars, this can be very helpful for you to take initiative with whatever you want. You can be excited and enthusiastic, ready to go.
When the transit planets are conjunct (aligned with) your natal Mars, your energy may be focused in a way that the transit planet directs. It can be strongest when it's transit Jupiter, Uranus, or Pluto. With the Sun, it can be physical, with Mercury, it can be mental, with Venus, it can be with others, with Saturn, it must be disciplined, and with Neptune, it can be spiritual.
When the transit planets are making a hard aspect (square and opposition) to your natal Mars, this can make it difficult for you to take initiative. You may lack energy, drive, and enthusiasm, or experience many blocks in the way that keep you from proceeding forward. You have to go at a slower pace, and make better plans.
Transit Mars brings great energy for taking initiative with the areas of life ruled by the house in your natal chart it's touring. This can be a period that lasts for about 2 months, and you may be extra driven to get started.
Transit Mars also aspects your natal planets, and when it's sextile or trine, you can find it's a lot easier to take initiative, and you have less in the way. When transit Mars is square or opposite one of your natal planets, this can be a challenging time for taking initiative, and you may lack energy and drive, have obstacles to overcome, and are generally pretty frustrated and need to have an outlet for energy.
Transit Mars conjunct one of your natal planets can make it important for you to take initiative with something that planet rules at that time. It usually only lasts for about 3-5 days, so it's something you can feel driven to take action with quickly. Without any action, you can be quite frustrated and lash out, so taking action is important.
You can feel this strongest with your Mars Return, which is when transit Mars returns to the exact same position it was at when you were born and conjuncts your natal Mars. This happens roughly every 2 years. You can feel extra energized and driven, and it can trigger you to take initiative with something.
When Mars is Retrograde:
A planet is retrograde when it appears to move backward from our view here on Earth, and retrograde planets operate differently since normal motion is forward. Mars retrogrades roughly every 2 - 2 1/2 years, and this can be a time when we have difficulty taking initiative. Personally, this may be focused on the areas of life ruled by the natal house(s) the Mars retrograde occurs in for you.
It can be strong with any natal planets Mars retrograde is square or opposite, but if Mars retrograde conjuncts, sextiles, or trines any natal planets, you may have an opportunity to take initiative for a second time, or in a different way than you've tried before.
Quick Note for Mars Retrograde Natives:
If you were born while Mars was retrograde, you're a Mars retrograde native. With Mars retrograde, you may struggle with taking initiative in your life, and may find that you have to learn how to do so at some point, otherwise you feel like you never really get anything off of the ground. You may have the best energy while Mars is retrograde in transit, and operate more smoothly, so that can be a time to look forward to.
Suggested Reading: The Expansion of Jupiter