The Rebbe's guidance on giving tzedakah regularly, and on the special times for giving:
Every day, even a little. Steady daily giving matters more than occasional large gifts — just as the body needs daily food (IK 2231). Making a habit of daily tzedakah, even in small amounts, refines a person and brings blessing (IK 3913). Giving tzedakah every weekday, especially on Mondays and Thursdays, was a practice the Rebbe often recommended (IK 2638), as was daily tzedakah for the merit of Rabbi Meir Baal HaNes (IK 1321).
Before prayer. Give tzedakah before praying each morning (IK 3542, 5038, 6259, 8225, 8376); it prepares one for prayer and draws added Divine help (IK 2886). The practice is well sourced, but it is not limited to that time alone (IK 8517). When one can't give on the spot, it can be put aside in a pushke (IK 3576).
Before lighting candles. Women have the old custom of putting aside tzedakah before lighting candles every Friday and before Yom Tov (IK 1592, 2638, 9492, 10301); the Rebbe especially encouraged it during pregnancy and for health and family matters (IK 1315, 1522, 1626, 5698).
Special times. Give Maos Chittim before Pesach — it brings personal freedom too (IK 961, 991, 3393). Give generously and with personal involvement on Chanukah (IK 9065, 9797), before Shavuos (IK 9500), and especially in Elul (IK 7429). On one's birthday, give tzedakah before prayer (IK 1548, 2097). There are customs about giving at night; follow them without being overly critical of others (IK 238, 4507). Giving at specially auspicious times brings particular blessing (IK 4511).
See also: Tzedakah – How much and how to give; Livelihood – Giving charity and maaser (tithing).